LumberJocks

Is polyurethane going to behave differently on harder woods?

  • Advertise with us

« back to Finishing forum

Forum topic by BobVila posted 12-18-2017 05:59 PM 2050 views 0 times favorited 15 replies Add to Favorites Watch
View BobVila's profile

BobVila

24 posts in 161 days


12-18-2017 05:59 PM

Thanks for the responses on my last post about bloodwood, very helpful.

She has asked for a matching set.

The clear satin polyurethane seems to stay tacky longer on the bloodwood. When I sand between coats I get a very odd feeling under the paper, it almost feels like little pieces of tacky polyurethane are rolling up (for lack of better description) under the sandpaper as I sand. Upon brushing it, it just doesn’t feel as dry as it usually does.

The last piece of bloodwood I finished did the same thing, it finished ok, looked fine. But I guess I’m just a little curious as to why this might be. This bloodwood is crazy, very dense wood, does that maybe have something to do with it? Temp and humidity are the same as always, 30% humid – 75 degrees F.

There is nothing to see in the picture, just figured I’d attach it.


15 replies so far

View LesB's profile

LesB

1683 posts in 3408 days


#1 posted 12-18-2017 06:11 PM

You did not indicate if the poly was water base or oil base. Also I have found that “old” poly seems to cure slower than fresh stuff.
I just made a large 18” platter out of Blood wood and coated it with General’s salad bowl finish and had no problem with it drying. Salad bowl finish is an oil and urethane product that drys fairly quickly and can be applied with a soft cloth or high quality paper towel (I use the blue shop towels).

-- Les B, Oregon

View MrUnix's profile (online now)

MrUnix

6590 posts in 2164 days


#2 posted 12-18-2017 06:31 PM

Softer, more porous woods will absorb more, therefore dry a bit faster. Also, the thicker you put it on, the longer it takes to cure. If you are sanding and not getting a fine white dust, then you need to let it dry longer. I gave up on brushing a long time ago, and now only do wipe on with a homemade mix (50/50). The first few coats dry within 15-20 minutes (or sooner), and there is no need to sand between every coat. You can also get almost any desired finish from a matte danish oil like sheen, to high gloss, and pretty much anything in between – your choice.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View BobVila's profile

BobVila

24 posts in 161 days


#3 posted 12-18-2017 06:49 PM


You did not indicate if the poly was water base or oil base. Also I have found that “old” poly seems to cure slower than fresh stuff.
I just made a large 18” platter out of Blood wood and coated it with General s salad bowl finish and had no problem with it drying. Salad bowl finish is an oil and urethane product that drys fairly quickly and can be applied with a soft cloth or high quality paper towel (I use the blue shop towels).

- LesB

Oil based. And yea, I know what you mean, that’s why I got a fresh bottle. In fact I really didn’t want to, I wanted to start saving money and buying more, but the larger can (less per ounce) didn’t get used fast enough, started crustin’ over.


Softer, more porous woods will absorb more, therefore dry a bit faster. Also, the thicker you put it on, the longer it takes to cure. If you are sanding and not getting a fine white dust, then you need to let it dry longer. I gave up on brushing a long time ago, and now only do wipe on with a homemade mix (50/50). The first few coats dry within 15-20 minutes (or sooner), and there is no need to sand between every coat. You can also get almost any desired finish from a matte danish oil like sheen, to high gloss, and pretty much anything in between – your choice.

Cheers,
Brad

- MrUnix

Yea, it dries pretty quickly, but I don’t get the dust yet, not even the next day. Never seen anything like it.

I normally do 2-5 coats, so you’re saying you don’t need to sand between? I’m just cautious I think because I’ve always sanded between coats and I’ve always had desired results, so I’m scared to change anything, ya know?

But I will say that I only sand with a 600-1000 grit, so it’s barely a sand, but does create dust.

View MrUnix's profile (online now)

MrUnix

6590 posts in 2164 days


#4 posted 12-18-2017 07:16 PM

As long as the poly hasn’t fully cured, you don’t need to sand… sanding is really only to 1) remove any nibs that may have got into the finish, and 2) give subsequent coats some tooth to hold onto when the previous coat has cured, as it’s strictly a mechanical bond (as opposed to something like lacquer which is a chemical bond and will melt into the previous coat). As long as you re-apply before the poly has a chance to fully cure, you really don’t need to sand, as it will be more or less a chemical bond at that point.

Are you brushing or doing a wipe-on? I will usually wipe on 3-6 coats one after the other without sanding in between (just letting them dry to the touch between each coat), then let it cure overnight. Next day, I’ll lightly sand it to remove any nibs or bumps and put on the final coat (or coats, depending on what kind of finish I’m looking for).

Not saying this is the proper or ordained method by the experts, but it works perfectly for me every time.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View BobVila's profile

BobVila

24 posts in 161 days


#5 posted 12-18-2017 08:16 PM


As long as the poly hasn t fully cured, you don t need to sand… sanding is really only to 1) remove any nibs that may have got into the finish, and 2) give subsequent coats some tooth to hold onto when the previous coat has cured, as it s strictly a mechanical bond (as opposed to something like lacquer which is a chemical bond and will melt into the previous coat). As long as you re-apply before the poly has a chance to fully cure, you really don t need to sand, as it will be more or less a chemical bond at that point.

Are you brushing or doing a wipe-on? I will usually wipe on 3-6 coats one after the other without sanding in between (just letting them dry to the touch between each coat), then let it cure overnight. Next day, I ll lightly sand it to remove any nibs or bumps and put on the final coat (or coats, depending on what kind of finish I m looking for).

Not saying this is the proper or ordained method by the experts, but it works perfectly for me every time.

Cheers,
Brad

- MrUnix

Hey, I’m really glad you said something. Ok, here’s my problem. Dust nibs. Anytime I sand, seems as if I have to put another coat on, I can’t fully remove the scratches & dust that the sanding causes, so I get stuck in a cycle and eventually just live with the flaws. No matter how fine of a grit I use, all the way to 2500. Once you’ve gotten the desired coats on, how are you polishing your finish to a smooth surface? I mean what is your final step in the process?

And to answer your question, in this case I’m using a foam brush, this is because I’m trying to keep the polyurethane out of the holes.

View MrUnix's profile (online now)

MrUnix

6590 posts in 2164 days


#6 posted 12-18-2017 08:32 PM

Last step is a final coat and leave it alone. With wipe-on, you rarely get dust nibs or other imperfections since the coats are so thin and basically rubbed smooth before allowed to dry. Sanding will produce scratches, so you need that last coat to fill them in and make them disappear. I also always use compressed air followed by a wipe down with MS after sanding just before applying the next coat. I also only use gloss poly, and tend to leave it glossy. If I wanted a satin or matte finish, then hitting it with some steel wool will give that effect. That to me is one of the great aspects of using gloss – you can make it look just about however you want with the proper technique.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View BobVila's profile

BobVila

24 posts in 161 days


#7 posted 12-18-2017 08:43 PM

Ok, I’ll try a thinner coat tonight.

My problem with the thin coats is I always feel it’s tough to get it coated evenly. I’ll have it on too liberally, so I’ll run the dry side of the brush over it. Then I notice a spot where I took off too much. I’ve gotta be making it more difficult than it is, can’t be rocket science.

View MrUnix's profile (online now)

MrUnix

6590 posts in 2164 days


#8 posted 12-18-2017 08:52 PM

My technique is similar to how a pimply faced high school student working the night shift at Wendys and really doesn’t want to be there wipes down the tables. Dunk the rag, plop it down and spread it all over in no particular pattern… just make sure the entire surface is covered. Then wring out the rag and go over it again, soaking up the excess and leaving a really thin wet film that will dry quickly. This way, you need to apply a lot more coats than if you were to brush – like 3 coats to equal one brushed on coat, but it works great and produces a very even finish.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View splintergroup's profile

splintergroup

1979 posts in 1188 days


#9 posted 12-18-2017 09:49 PM

Some woods exude more oils than others, even among the same species.

Case in point, poly on jatoba. It never really dried. I sanded it all off but the wood still felt greasy. A wipe down with lacquer thinner seemed to clean it right up and the poly (for the same can) acted and dried like it should.

View JBrow's profile

JBrow

1348 posts in 885 days


#10 posted 12-18-2017 10:01 PM

BobVila,

I typically finish with oil-based urethane or polyurethane, which seem to behave similarly when applied. My protocol is to start with the workshop. I spend a couple of hours vacuuming the garage workshop trying to eliminate as much dust as possible and hopefully reduce the need for between coat sanding. I do not have a separate room dedicated to finishing. Vacuuming the shop is intended to reduce the dust that may become air borne when the garage door is cracked to allow off-gassed fumes to escape. Once the shop is vacuumed, the work piece is vacuumed and any air borne dust is allowed to settle overnight.

The next day, the project is wiped with a tack cloth until a fresh tack surface is clean. The first coat of finish is applied with a foam brush and the project sets overnight. The garage workshop in the winter is 65 to 70 degrees.

The following day the first coat of finish is more or less rough to the touch probably from settling dust and raised wood fibers. The project is sanded with the grain using 220 grit sand paper and light pressure. A white fine dust is produced. There is some, but minimal, clogging of the paper. I found that higher grits of sand paper seem to clog easier. The finish fills the scratches from 220 grit paper and leave a smooth level surface. The project is then vacuumed and tacked. The second and third coat of finish is applied. The third coat is applied based on the minimum re-coat period specified by the manufacturer.

I never return unused finish to the can. I fear that any dust picked up by the foam brush could contaminate the dip pot (from which I re-load the brush) and thus could contaminate the finish in the original can.

The finished project is mostly smooth to the touch with only an occasion nub. I generally declare the project done at this point.

I used above described protocol when finishing a bloodwood countertop with 5 coats of urethane. It was smooth to the touch after the last coat.

While I generally declare the project done at this point, a final rubbing of the project could be done using an abrasive pad. I am not sure how scratches left in the finish by abrasive pads compare to scratches left by steel wool. Here is a brief explanation of abrasive pads…

https://www.woodmagazine.com/materials-guide/sanding/color-holds-the-key-to-abrasive-pads

On my upcoming project I will introduce an additional step. I will wet the surfaces of the project with water to raise the grain and then do my final pre-finish sanding at 180 grit. This may reduce the roughness after the first coat of finish.

View BobVila's profile

BobVila

24 posts in 161 days


#11 posted 12-19-2017 03:09 PM


Some woods exude more oils than others, even among the same species.

Case in point, poly on jatoba. It never really dried. I sanded it all off but the wood still felt greasy. A wipe down with lacquer thinner seemed to clean it right up and the poly (for the same can) acted and dried like it should.

- splintergroup

Bob Vila Likes this


BobVila,

I typically finish with oil-based urethane or polyurethane, which seem to behave similarly when applied. My protocol is to start with the workshop. I spend a couple of hours vacuuming the garage workshop trying to eliminate as much dust as possible and hopefully reduce the need for between coat sanding. I do not have a separate room dedicated to finishing. Vacuuming the shop is intended to reduce the dust that may become air borne when the garage door is cracked to allow off-gassed fumes to escape. Once the shop is vacuumed, the work piece is vacuumed and any air borne dust is allowed to settle overnight.

The next day, the project is wiped with a tack cloth until a fresh tack surface is clean. The first coat of finish is applied with a foam brush and the project sets overnight. The garage workshop in the winter is 65 to 70 degrees.

The following day the first coat of finish is more or less rough to the touch probably from settling dust and raised wood fibers. The project is sanded with the grain using 220 grit sand paper and light pressure. A white fine dust is produced. There is some, but minimal, clogging of the paper. I found that higher grits of sand paper seem to clog easier. The finish fills the scratches from 220 grit paper and leave a smooth level surface. The project is then vacuumed and tacked. The second and third coat of finish is applied. The third coat is applied based on the minimum re-coat period specified by the manufacturer.

I never return unused finish to the can. I fear that any dust picked up by the foam brush could contaminate the dip pot (from which I re-load the brush) and thus could contaminate the finish in the original can.

The finished project is mostly smooth to the touch with only an occasion nub. I generally declare the project done at this point.

I used above described protocol when finishing a bloodwood countertop with 5 coats of urethane. It was smooth to the touch after the last coat.

While I generally declare the project done at this point, a final rubbing of the project could be done using an abrasive pad. I am not sure how scratches left in the finish by abrasive pads compare to scratches left by steel wool. Here is a brief explanation of abrasive pads…

https://www.woodmagazine.com/materials-guide/sanding/color-holds-the-key-to-abrasive-pads

On my upcoming project I will introduce an additional step. I will wet the surfaces of the project with water to raise the grain and then do my final pre-finish sanding at 180 grit. This may reduce the roughness after the first coat of finish.

- JBrow

Ok thanks. I saw an article from Woodworkers that said to use turtle wax, not sure if that works.


My technique is similar to how a pimply faced high school student working the night shift at Wendys and really doesn t want to be there wipes down the tables. Dunk the rag, plop it down and spread it all over in no particular pattern… just make sure the entire surface is covered. Then wring out the rag and go over it again, soaking up the excess and leaving a really thin wet film that will dry quickly. This way, you need to apply a lot more coats than if you were to brush – like 3 coats to equal one brushed on coat, but it works great and produces a very even finish.

Cheers,
Brad

- MrUnix

Ok, yea I know what you mean, I noticed last night I seem to get desired results by wiping off excess with a rag that has some on it, almost like using a shammy. Seems to help.

View splinter164's profile

splinter164

19 posts in 1907 days


#12 posted 12-19-2017 03:31 PM

Two thumbs up for a serious vacuuming and letting dust settle before finishing in my one room shop. One other trick (that I think I learned from this site) is to use a spray bottle to mist the floor before starting a coat of finish. It helps to stick down the dust I might stir up while walking around a large piece. Takes 5 mins and the only downside is my wife asking where the sprayer she uses for ironing has disappeared to.

  • from JBrow – I spend a couple of hours vacuuming the garage workshop trying to eliminate as much dust as possible and hopefully reduce the need for between coat sanding. I do not have a separate room dedicated to finishing. Vacuuming the shop is intended to reduce the dust that may become air borne when the garage door is cracked to allow off-gassed fumes to escape. Once the shop is vacuumed, the work piece is vacuumed and any air borne dust is allowed to settle overnight.*
View OnhillWW's profile

OnhillWW

129 posts in 1197 days


#13 posted 12-19-2017 05:02 PM

Just to reinforce splintergroup’s point. Oily woods get a prefinish wipe down with either mineral spirits, naphtha, or lacquer thinner as close to the time I will apply the first finish application allowing for drying flash off. This draws out oils and pulls off dust. Doing this and waiting too long before application can allow natural oils to migrate from inside the wood back to the surface. And of course allowing for through cure time before sanding between coats.

-- Cheap is expensive! - my Dad

View BobVila's profile

BobVila

24 posts in 161 days


#14 posted 12-20-2017 07:48 PM



Two thumbs up for a serious vacuuming and letting dust settle before finishing in my one room shop. One other trick (that I think I learned from this site) is to use a spray bottle to mist the floor before starting a coat of finish. It helps to stick down the dust I might stir up while walking around a large piece. Takes 5 mins and the only downside is my wife asking where the sprayer she uses for ironing has disappeared to.

  • from JBrow – I spend a couple of hours vacuuming the garage workshop trying to eliminate as much dust as possible and hopefully reduce the need for between coat sanding. I do not have a separate room dedicated to finishing. Vacuuming the shop is intended to reduce the dust that may become air borne when the garage door is cracked to allow off-gassed fumes to escape. Once the shop is vacuumed, the work piece is vacuumed and any air borne dust is allowed to settle overnight.*

- splinter164

Yea, I’ve never vacuumed before, I will give it a try. Maybe I should turn off the dehumidifier a few hours beforehand too, maybe it could help eveything settle. I thought about draping a plastic curtain around that area. It’s so dry in my basement I don’t think it’d effect dry times (or anything else).


Just to reinforce splintergroup s point. Oily woods get a prefinish wipe down with either mineral spirits, naphtha, or lacquer thinner as close to the time I will apply the first finish application allowing for drying flash off. This draws out oils and pulls off dust. Doing this and waiting too long before application can allow natural oils to migrate from inside the wood back to the surface. And of course allowing for through cure time before sanding between coats.

- OnhillWW

BobVila likes this

View William Shelley's profile

William Shelley

564 posts in 1435 days


#15 posted 12-20-2017 11:18 PM

Oily hardwoods seem to be problematic for more than just polyurethane.

I used 100% tung oil on Bloodwood and after a month or so I started to notice weird white patches that could be wiped off the surface but still kind of stuck in the grain. I think it was a reaction from the wood oils and the tung oil I applied.

Next time, I will wipe down with alcohol or acetone.

-- Woodworking from an engineer's perspective

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com