The first application of polyurethane finally dried on my red oak baseboard. I sanded the baseboard and used a tacky cloth to remove the dust. Should I wipe down the baseboard with a rag dipped in mineral spirits before I apply the next coat of polyurethane?
Thanks. Sandy
Why would you want to do that? I certainly never have.
No. Poly uses a mechanical bond between coats, you want as little as possible between coats so as to get the best bond. Just make sure you get the dust off, that's all.
And I hope by "finally dried" you mean "I waited as long as the can said to wait". If you don't recoat in the time specified, you need to wait at least a week or two (to fully cure) and sand before recoating. If you recoat in the time specified, you only need a light sanding (to remove raised grain and such) as the poly hasn't fully cured so it will bond with the next layer whether you sand or not.
Once cured there is nothing wrong with wiping down with mineral spirits. I have done this with as little as 8 hours curing time.
B.S. You can sand after 24hrs. I usually use 320 grit wet/dry paper and lightly sand.
So are you saying that you can only sand during the time specified on can (3 hrs usually) and after one week? This makes no sense.
Sure. There is nothing wrong with doing this. As long as you wait long enough for the poly to cure sufficiently (varies by temperature/humidity).
If you were able to sand successfully, than your poly was cured sufficiently. There is no danger in wiping sufficiently cured poly with mineral spirits. I have done this NUMEROUS times.
After all, most people use mineral spirits as a solvent to dilute polyurethane. How could this same solvent be counter indicated for wiping cured poly? :)
Go right ahead!
Stoutman,
The mineral spirits will remove any residue left behind by the tack cloth. Nothing wrong with doing this if the poly has cured sufficiently.
Therefore it's not rational?
Then wipe it down with alcohol to get rid of the solvent reside.
Pete
Cure time depends on the poly. The stuff I use isn't cured until at least a week passes. After 8 hours it's only dry, but not cured.
Well, nobody is going to stop you if you want to sand whenever you feel like it. I'm assuming the OP wants as good a finish as is practical.
I've noticed that before the recoat time, I can sand lightly, and only to remove raised grain. The poly is still soft enough that if I try to sand into it, it just rolls up into lumps (like a pencil eraser). Only after it's fully cured is it hard enough to actually sand into the poly.
When fully cured, sanding poly should result in a fine, light, dusty powder. Like what you usually see with sanding sealer.
I'm saying that if you recoat outside of the recommended times, the coat won't hold as well. Why? Because of how the poly layers join. During the cure time, the poly molecules go from loose to tight, so if you recoat during that time you can still hook into the uncured molecules for a mechanical join at the molecular level. Once they're cured too much, this is no longer an option for a reliable join, so you wait until it's cured enough (i.e. hard) to sand it to make a mechanical joint at the macro level.
The curing process starts after it is applied. There are degrees of 'cured'.
24 hrs is plenty of time to lightly sand with 320 wet dry.
What do you think I am talking about here? Re-read what I wrote above.
When it behaves like this during sanding your poly has not 'sufficiently' cured. Granted curing time is governed by humidity and temperature.
Wrong. It does NOT need to be "fully cured", only sufficiently cured.
I get the same results when 'sufficiently' cured. Doesn't have to be "fully cured".
Sanding is usually required after 24 hrs.
Feisty woodworkers up in here! 😄
I think the question is, is it recommended to clean sand dust with mineral spirits before applying next coat?
I sand lightly and clean. No mineral spirits are necessary. At least, that's what I do, and it comes out very well.
A tack cloth is probably better than mineral spirits, poor quality solvents are likely to cause finishing issues.
A tack cloth is useful to remove the sanding dust.
I use Festool sanders with a dust extraction so there is little to no dust left on the surface, NOW.
Before dust extraction I simply used compressed air to remove the dust. That worked very well.
Mineral spirits works well to remove anything missed by the sanders with dust extraction. Mineral Spirits gives you a preview of what the wood is going to look like with varnish applied and evaporates relatively quickly and leaves no residue nor does it raise the grain.
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