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Seems I read somewhere that when using wipeon poly, since it contains mineral spirits, that the last top will kind of melt into the new coat when you wipe the last one dry, that no sanding is needed. Is this true?
I've discovered that I really like using it. Wanted to add extra coats over pieces finished long ago.
Read the label. Most all I ever used, oil base , wanted you to scuff smooth with steel wool.
If nothing else, it will take out any nibs and make it really smooth.
Some water base you can recoat with out sanding within a certain time, after that you needed to sand before recoat.
What product are you using?
Oil Based Varnish in including polyurethane resin based products do not generally "burn in". There is no "melting" between coats with these products like there is with shellac, solvent based lacquer and some water borne products.
The issue with sanding between wipe on coats is that they are extremely thin coats, so in general, you'll only want to sand with very fine abrasives if your last coats were applied beyond the re-coating window listed in the product instructions. That's usually something like the next day. This is to insure that new coats can adhere to the previous coats. Repeated coating during the same day shouldn't need to be abraded with most of these products, but again...read the instructions for the product you are using.
Minwax wipeon poly (oil base).
I've used Minwax wiping poly a few times and sanding even lightly would cut through and show layers - no burn in as far as I could tell.
"when using wipeon poly... the last top will kind of melt into the new coat when you wipe the last one dry, that no sanding is needed. Is this true?"
This is true only when wiping on coats after the first has DRIED, but not CURED. Usually, this means wiping on in about an hour. In this case the top layer does re-dissolve the second layer and they can form a single layer.
"since it contains mineral spirits".
This is not the reason. There is nothing magical about mineral spirits; all solvents (ethanol even water) will dissolve the finish if it hasn't had a chance to fully cure. Drying is simply the evaporation of the solvent. Curing is a chemical reaction to the resin that causes it to permanently harden and bond with itself. If you get the original solvent on there before the curing starts, you can even REMOVE the finish.
"Wanted to add extra coats over pieces finished long ago."
This may be possible, but your new layer will not fuse with the existing layer; it will be deposited on top. Any kind of oil or residue can prevent the poly from bonding properly, so you should at least wipe the surface with mineral spirits, and best prime the surface with dewaxed shellac. This is assuming the original finish was a film forming finish, not a softer, oil finish, and that the surface was truly finished long, long ago, such that you can be sure that the previous varnish has completely cured. This is not complete advice.
The reason to wipe on successive coats quickly (and beware that you shouldn't do more than 2-3 this way before giving the surface a good time to dry before applying more coats) is to speed overall application and to provide a slightly more suitable surface (thicker) for possible rubbing out. However, I find this fine to do for the initial coats before build is achieved. But once it starts to build, I prefer to wait until the coat is fairly hard and not tacky at all before applying the next. I think it lets me move quicker, and thinner and which promotes a dust-free application which means very minimal sanding with fine abrasives every 3-4 coats.
I'm a bit low on the learning curve. Please explain what you mean by "burn in".Quote:
Originally Posted by Stan Calow
Nevermind,Stan. I hadn't noticed that Jim had already defined it.
- Apply a liberal amount of Wipe-On Poly to a clean, soft, lint-free cloth and rub it into the wood.
- Let dry 2-3 hours. Then lightly sand entire surface with fine sandpaper (220 grit) to ensure an even finish and proper adhesion. Remove all dust.
- Apply a second coat. If a third coat is desired, repeat step 2 before application.
- After final coat, allow 24 hours before light use
I might add that I think almost every finish benefits from some sort of between-coat rub-down. Like Jim mentioned, at minimum, it gets the dust nibs out of the finish.
TedP