I am coating a pine mantlepiece with gloss polyurethane. I used interior/exterior poly for my 1st & second coats for protection from sun exposure. Because of the longer drying time 24/48 hours there are quite a bit of dust nibs. Can I use a faster drying interior poly (2 hours) as my top coat or are the two types incompatible?

  • Hi, welcome to StackExchange. Just in general, it's really impossible to answer this definitely, because you might have problems but it depends on a host of specifics. The easiest solution here is to continue to use your interior/exterior poly and simply dilute it, so you can apply it as wiping varnish. This almost completely obviates problems with dust nibs, and simultaneously make it dry faster (thinner coat, ergo faster drying time) but of course you need more coats to build an equivalent finish.
    – Graphus
    Dec 29 '20 at 17:11
  • BTW just to check, is the fast-drying interior poly also an oil-based varnish??
    – Graphus
    Dec 29 '20 at 17:11

Since it has already been a day since your last coat coat, the polyurethane should be done drying (along with some curing) so you can safely apply another coat of polyurethane if it has the same base (oil/water).

There might be some issues if you apply different polyurethane (with the same base) within 2 hours since there will be chemical bonding between the two layers but that seems unlikely.

If more than 2 hours have passed but less than a day has gone by you should also avoid new coats since the difference in curing between coats might cause some weird patters to show up once it's all done curing.

If you are mixing between bases (oil/water) ideally you would wait a month, or more. That way you can be sure that it's done curing and won't have any interactions between the different polyurethane layers. Since it's been a few days you might be able to get away with just applying the new coat after some light sanding with 220 sandpaper (I would do this anyway to knock down those dust spots). However, there is chance you might get a weird texture/cracking in the finish.

  • I agree. I have put oil-base over water-base and vice-versa after base coat was dry, with no problems. Definitely shouldn't be an issue if both are the same type. Lightly sand to degloss, clean with tack cloth, use professional grade brush, good to go. Dec 31 '20 at 3:33
  • You really shouldn't apply a waterbased topcoat over something oil based until the former has had a good long time to cure, and with varnishes and oil/varnish mixes the rule of thumb is until the characteristic odour can't be detected any more. And actually it's the same with leaner/faster drying finishes over a noticeable coat of something that 'dries' more slowly, otherwise defects can occur because the surface dries, contracts and begins to harden while what's underneath is not finished being mobile and shrinking.
    – Graphus
    Dec 31 '20 at 8:48
  • Graphus I agree that you should wait until the oil based poly is completely cured before applying another base, However I was under the impression this was due to the curing not the drying since most of the solvent is gone within a few hours. Whoever I've used oil bases poly, that smell was gone in like a week, then again I had good ventilation and didn't get next to the wood to try to smell it. .
    – Mendoza
    Jan 6 at 1:24
  • Yeah the standard for 'it doesn't smell any more' is apparently to put your nose right on the surface when sniffing (source, Bob Flexner). This is really only concerned with something faster-drying over an oil-based varnish though, one could probably get away with trad varnish over a waterbased one after just a couple of days (although waiting a week+ would be ideal). BTW you have to use the @ before someone's username for them to get a notification of a reply.
    – Graphus
    Jan 7 at 10:18

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