Help with window sill refinishing

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Forum topic by ron66 posted 10-18-2010 10:40 PM 19725 views 0 times favorited 7 replies Add to Favorites Watch
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3 posts in 2800 days

10-18-2010 10:40 PM

I have two large bay window sills that need refinishing from water damage, sun, and drying out, etc. There are numerous cracks in the oak veneer, along with discoloring from water damage, both from direct water contact (rain, flower pots, etc. and also winter condensation). The sills are about 4 years old. I plan to sand the sills down to new wood, then stain, then top coat. What would be the best type of top coat finish to apply for a situation like this. I’ve gotten different advice, from marine finish to spar urethane to polyurethane, and am now really uncertain which way to go. Also, with the numerous cracks in the veneer, will the top coat fill these adequately, or should I do something else. Any advice is appreciated.

7 replies so far

View Brit's profile


7385 posts in 2866 days

#1 posted 10-19-2010 12:01 AM


First of all welcome to LJs, there is a wealth of experience here and I’m sure you will enjoy it. Secondly, can I ask you to clarify a couple of things as it makes a difference to the advice you might get about a finishing regime.

1) When you say you plan to sand back to new wood, does that mean that you are going to:

a) lightly and carefully sand the existing veneer to freshen it up.
b) sand right through the veneer and finish the substrate underneath the veneer. If so, do you know what is under the veneer?
c) sand through the veneer and apply new veneer before refinishing.

2) Is there any chance you could post a picture showing the extent of the damage?

3) Will there continue to be the prospect of water damage to the refinished window sill. If so, it limits the kind of finish that can be applied.

4) What kind of a finish are you after. Do you want a close to the wood finish such as boiled linseed oil or danish oil or do you want the surface to be glossy and glass-like when you’re done?

Let us know and then we can offer some suggestions. A picture would definitely help though.

-- - Andy - Old Chinese proverb says: "If you think something can't be done, don't interrupt man who is doing it."

View ron66's profile


3 posts in 2800 days

#2 posted 10-19-2010 02:00 AM

Thank you Brit for the reply. A photo is out since I don’t have a working camera at present. As far as sanding, my intent would be to remove the top coat and sand into the veneer just enough in order to restain, as the existing stain has a noticeable amount of fadeing. The existing top coat has a matte or satin type finish. I believe a Minwax product was used, likely a polyureathane type. My intent would be to achieve that type of finish again in order to match the rest of the trim. The water damage I refer to has mostly been due to carelessness (leaking flower pot, leaving windows open during rain, etc.). While I will strive to prevent this in the future, it is probably unavoidable to have some water contact with the finish at some point in time. However, as I mentioned, there has been a lot of cracking in the veneer in areas which have’nt had any moisture contact. I presume this could be due to the wood drying out during winter, and also some sun damage. Some of the cracks as pronounced enough that they appear to extend well into the veneer material. Hope this additional explanation helps.

View shipwright's profile


7992 posts in 2821 days

#3 posted 10-19-2010 03:07 AM

You could, after staining, seal with an epoxy, sand and then apply the cosmetic finish of your choice. It would be bulletproof and would put an end to the checking.

Paul M

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

View Brit's profile


7385 posts in 2866 days

#4 posted 10-19-2010 03:48 AM

Ok, as I said it is difficult to say for sure without seeing it, but here goes:

Veneer is usually very thin, so sand very carefully. You are unlikely to remove all of the watermarks, but you should at least be able to improve it. Use a block and don’t apply too much pressure. Sand with the grain to 220 grit. Vacuum the surface and then lightly wipe the surface with a tack cloth to pick up any remaining dust from the pores. You’ll be amazed at how much a tack cloth picks up that the vacuum doesn’t. If you intend to use a water-based stain, don’t use a tack cloth. With a spiritpbased stain, the tack cloth is fine.

If the veneer has lifted where it has cracked, you will need to carefully lift the edges of the cracks and glue it back down. I would apply masking tape right up to the edges of the crack to avoid getting glue anywhere I didn’t want it. Once you get it glued, place a heavy weight over it with sellotape on the underside of the weight to prevent it getting stuck to the window sill. Once dry, you will need to fill the cracks. If they are just thin splits, you can use a gap filling CA glue to fill the gap. The Titebond Medium CA glue is probably the one to go for. Once you’ve applied it, remove the masking tape from around the crack immediately. When it has thoroughly hardened level it off with 220 grit using a sanding block.

Now you are ready for your stain. Basically, follow the directions on the tin, but when you are doing a sizeable surface like a window sill, it is best if you can work quicky and keep a wet edge as you move along. Since you need to work quickly, mask up anything you don’t want to get stain on. Try to get it a nice even colour.

Now for the top coat. I would go with a polyurethane varnish. Use a good brush and apply two thin coats brushing with the grain. You will inevitably get dust settling in your finish, but on an open grained wood like oak it shouldn’t be that noticable as the surface won’t be glassy smooth unless you fill the grain after your initial sanding, but that is probably a step too far. Resist the temptation to apply more coats because it will start to look worse on an open-grained wood. Don’t put anything heavy on it for a couple of weeks.

Good luck.

-- - Andy - Old Chinese proverb says: "If you think something can't be done, don't interrupt man who is doing it."

View Dark_Lightning's profile


3165 posts in 3132 days

#5 posted 10-19-2010 04:00 AM

What Brit said, with this caveat- you will likely have to clean up and refinish your sills every year, from the sound of it. Waiting 4 years was too long, obviously. The ultraviolet in sunlight is brutal. Even spar varnish takes a beating from it. That is, if this is the outside portion of the sills.

-- Random Orbital Nailer

View KnotWright's profile


258 posts in 3511 days

#6 posted 10-19-2010 04:15 AM

Since time is always money, for me the quick, easiest fix for this would be just to install a nice new piece of 1/4” oak plywood over the damaged one and just put a nice new coat of Spar varnish, MinWax makes a good one, and several other quality brands are out there. Most all of them now come with UV protection built into the formula. Several thin coats should provide you with many years of enjoyment.

The new layer of plywood can be glued down and tacked in place with some small finish nails.

I had a bay window that had many, many years of paint build up on it and I really don’t care much for sanding and refinishing, so I opted to just cover over the offending painted version. Put on a nice red oak piece of 1/4” plywood along with some new trim on the front, and varnished it, came out looking great.

-- James

View ron66's profile


3 posts in 2800 days

#7 posted 10-20-2010 01:53 AM

Thank you all for the good information and ideas. I will need to think about this for a while and then just do it and hope for the best.

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