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Polyurethane Spray too thick and runny. Please help...

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Forum topic by SchwaNa posted 05-21-2018 06:00 PM 2117 views 0 times favorited 16 replies Add to Favorites Watch
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SchwaNa

6 posts in 87 days


05-21-2018 06:00 PM

Topic tags/keywords: finish polyurethane guitar oil spray

Hello All, Total newbie here. I’m working on my first wood project. A Mahogany guitar body. I shaped and contoured and sanded filled the grain and sanded again and was very happy with it. For me this is about the learning and the experience, I don’t intend to sell it so not afraid to experiment and make mistakes, which I have!
I put a number of thin coats of Tru-Oil (gun stock finish) and loved it. Really made the mahogany shine. a few weeks later, decided to put on a poly finish. I’m still not sure why. So I got one of the aerosol type containers and sprayed (after researching and watching plenty of videos).
Here’s my problem. It covers the whole body but very uneven. In some places there’s a lot of build up and spills and runs, also the stickiness catches a lot of particles and it is very bumpy to the touch. It does not look good.
What can I do now? Sand the whole thing back down? or just a touch? Will I be able to sand it down once it’s dry?
Please be specific about what to use, I would like to have a protective layer of poly but it needs to be clear and smooth.
Thanks a lot!


16 replies so far

View MrUnix's profile

MrUnix

6845 posts in 2281 days


#1 posted 05-21-2018 06:03 PM

I’d let it cure, then sand it down to smooth (no need to completely remove). Then would use some wipe on poly instead of rattle cans.

Cheers,
Brad

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

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SchwaNa

6 posts in 87 days


#2 posted 05-21-2018 06:18 PM

Thanks Brad,
If you don’t mind some follow ups:
Sand dry or wet?
If wet – with what? (lemon oil? spirits? water?)
and what grit?

You don’t think I’ll have the same problem after a coat of wipe-on? How can I make sure that the final coat is “perfectly” even?
Sorry if this is all elementary stuff.

View Rich's profile

Rich

3340 posts in 671 days


#3 posted 05-21-2018 06:49 PM

Guitars beg for lacquer. Even with an aerosol spray you’ll get perfect results.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View Ripper70's profile

Ripper70

1085 posts in 990 days


#4 posted 05-21-2018 07:12 PM

Nitrocellulose lacquer is the professional standard for guitar finishes. You can try this or another like it. StewMac is a good resource.

-- "You know, I'm such a great driver, it's incomprehensible that they took my license away." --Vince Ricardo

View the_other_ken's profile

the_other_ken

33 posts in 3057 days


#5 posted 05-21-2018 07:35 PM

I’d just like to add to what Brad said. If you have runs, drop or any bumps on the surface, you should level those before you start sanding. Use a razor blade, chisel or similar and carefully remove those raised surfaces by carefully scraping or paring them down. Be careful and don’t take it down to bare wood. It needs to be fully cured to do this and those lumps will take longer to cure.

Then you can sand the surface level with 220 or finer grit and try to refinish. If you wet sand, just use water with maybe a drop or two of detergent. I would just dry sand it. The easiest finish would be a wipe on poly.

View LesB's profile

LesB

1790 posts in 3525 days


#6 posted 05-22-2018 07:04 PM

You said you were not sure why you decided to use the Poly. I asked myself that same question when you said you had a nice finish with the Tru-Oil. I have used Tru-Oil on several gunstocks and it produces a fine durable finish. One of the things I like about it is that it can be easily retouched if there is wear, scratches or other damage.
Tru-Oil is a linseed based product with dryers added for a faster cure. It cures (it does not “dry”) on exposure to Oxygen and forms a moderately hard durable finish.

-- Les B, Oregon

View Sark's profile

Sark

74 posts in 442 days


#7 posted 05-22-2018 09:05 PM

First thought: poor spraying technique. I’ve done a lot of spraying in my cabinet shop using high quality equipment, but I’ve screwed up lots of project with a spray can. A pressured can with finger pressure is tough to control without getting too thick in one area.

Second Thought: Polyurethane not sticking properly to the oiled finished. This would make it run/drip more than if you sprayed on raw wood, especially if you didn’t sand the oiled wood.

Third Thought: A bad can of polyurethane. Actually, I just reread your post, and I’m thinking this is the most likely cause. I have had this happen to me plenty of times. Especially when buying from a local store where the can probably has been sitting on the shelf for a long time. Or where its just a cheap lousy product. The difference between say what HD sells and what professionals use is huge. I use Mohawk for aerosol lacquer and sealer. (Continued on next post)

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Sark

74 posts in 442 days


#8 posted 05-22-2018 09:23 PM

And how to fix the problem: I think you will need to refinish, unfortunately. Gummy finish may never dry out. I screwed up once with a gummy finish on a drawer, and 10 years later it was still gummy. You’ll need a stripper. With luck you can take off the poly without mucking up the underlying oil. Use copious amounts of orderless paint thinner after stripping and make sure that the wood is really really clean and dry. Re-oil the wood if you need to, but hopefully you won’t need to. Nor will you need to sand anything.

For topcoating, you may find that rub-on wipe-off finish is easier to use than spraying. With a wipe on finish, you can get the coating extremely thin and free of any brush strokes. See if you can buy oil-based polyurethane based wiping finish. Minwax is available at Home Depot, but I would avoid. It’s pretty mediocre product. And use gloss. It’s easy to take the gloss out of a finish if its too shiny but hard to add gloss to a flattened finish. And you must sand between layers since poly will not stick to itself. Use really fine paper or sanding sponges with 600 grit. Good luck.

View Rich's profile

Rich

3340 posts in 671 days


#9 posted 05-22-2018 09:29 PM


Pre-catalyze lacquer will not stick to itself, so it must be sanded.

- Sark

Total BS. I use Mohawk and SW pre-cat lacquers and that is simply wrong.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View SchwaNa's profile

SchwaNa

6 posts in 87 days


#10 posted 05-23-2018 02:22 AM

Hi Everyone,
Thank you all so much for taking the time to read and comment about my post. I learn so much just from reading your suggestions here.
I do everything by hand so it takes time and elbow grease but I’ll report slowly about my progress (or lack thereof..)
OK – damage assessment: It didn’t look good when it dried. not horrible but not pretty. very uneven. I used Varathane brand poly spray (gloss) from HD. So I sanded it a bit with 600 just to see if I can get it level (I couldn’t). I think I’ll try to strip that finish and use another method.
stripping: Is citristrip good for this work? Will it remove all the oil as well?
Cleaning: Paint thinner? Is mineral spirits ok? again – will it rub off the oil?
and for the top coat – should I leave it just with the truoil? or should I do wipe on poly?
also – couldn’t understand the difference between the above mentioned ‘pre-catalyzed clear lacquer’ and the ‘tone finish clear lacquer’, both from Mohawk.

Thanks again, I have a different question about finishing a table top but I’ll open another post.
Best,
Ido

View Sark's profile

Sark

74 posts in 442 days


#11 posted 05-23-2018 02:43 AM

Rich, that’s rather harsh. How about just saying you disagree?

View Rich's profile

Rich

3340 posts in 671 days


#12 posted 05-23-2018 04:05 AM


Rich, that’s rather harsh. How about just saying you disagree?

- Sark

It wasn’t harsh, and disagreements are about opinions, not facts.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

View TungOil's profile

TungOil

987 posts in 577 days


#13 posted 05-24-2018 12:18 AM

+1 on lacquer. Most true lacquers will burn in to the previous coats, effectively forming one thick layer when finished. Not all water bourne lacquers will do this but most solvent based will. It’s easy to apply but really must be sprayed due to the fast dry time.

without a true spray booth you will have a hard time eliminating all of the dust nibs in the finish. The fast dry time of lacquer will work in your favor here, so another good reason to use it. In the end, I have found this little gadget priceless for denibbing:

http://lumberjocks.com/reviews/10297

good luck!

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

View SchwaNa's profile

SchwaNa

6 posts in 87 days


#14 posted 05-24-2018 02:51 AM

I’m a bit hesitant about getting the spray on lacquer (nitro) – especially after the poly spray fiasco.
I’m asking this again:
stripping: I’m going to try and strip the spray coat. Is citristrip good for this work? Will it remove all the tru-oil as well?
Cleaning: Paint thinner? Is mineral spirits ok? again – will it rub off the oil?
and for the top coat – I understand that tru oil is not enough, correct? So I should do some wipe on, like lacquer or poly. and that’s not all – oil based or water based? home made cocktail or pre made from homedepot?
Thanks everyone,
Ido

View Sark's profile

Sark

74 posts in 442 days


#15 posted 05-24-2018 03:28 PM

OK, I’ll answer the easy part first. The vinyl sealer sticks tight to many types of materials. Lacquer sticks to the sealer just fine. Which is why the sealer is used as the first coat. Lacquer is inherently more finicky about what it gets sprayed onto. So first the vinyl sealer. If you follow the recommendations on the can, you sand between coats. Add enough coats and sand till it’s smooth and nice looking and you have the thickness of finish you desire

Next is the top coat, which is the pre-cat lacquer. I sand between coats. It may not be necessary, as Rich states, but I always do. One of the great advantages of using lacquers is that they can be sanded and polished. Tough to do with polyurethane and oiled finishes. You don’t need to worry about dust nibs too much. They can be polished away after all is dry. Or use the tool TungOil suggests. Never used it, but it looks pretty neat.

I haven’t done enough stripping to recommend a particular product. I think you’re on your own here. Citristrip should work, but if it doesn’t you’ll need to use a more potent stripper. I don’t think the mineral spirits will take out your oil, but if it does, just recoat with oil. Oil finishes aren’t generally used on guitars but since you started that way, I think you have to stick with it, let it dry, and apply a decent top coat.

I would use a spatula (maybe a plastic one) to scrape off the old finish. Then rags. The rinse with whatever Citristrip recommends. (I don’t know if its water based or oil based chemicals) I like (and have had success with) de-odorized mineral spirits. Good luck.

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