Minwax Poly - Brush on vs. Aerosol

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ErnieM

Ernie
Corporate Member
I sometimes use Minwax Poly in a rattle can for small parts. The directions say to recoat within 2 hours or wait a minimum of 72 hours, which in my opinion, is ridiculous. I've tried recoating in less than 72 hours, and it usually results in a wrinkled finish. The brush on Minwax Poly has no 72 hour waiting period between coats, and doesn't wrinkle when re-applied in a more reasonable time frame. I'm wondering what it is about the aerosol poly that requires such a long time between coats. I've read that the aerosol is thinned with acetone, while the brush on version is thinned with mineral spirits. Is that what causes the difference? I find it amusing that the aerosol is labeled as "Fast Dryiing Polyurethane" but the recoat time is anything but.

This question is asked more out of curiosity than anything else. I've gotten around the problem by thinning the brush on Poly and spraying these small parts using an air brush. Or, I just spray lacquer.

Can anyone shed some light on this?

Ernie
 

CrealBilly

Jeff
Senior User
I sometimes use Minwax Poly in a rattle can for small parts. The directions say to recoat within 2 hours or wait a minimum of 72 hours, which in my opinion, is ridiculous. I've tried recoating in less than 72 hours, and it usually results in a wrinkled finish. The brush on Minwax Poly has no 72 hour waiting period between coats, and doesn't wrinkle when re-applied in a more reasonable time frame. I'm wondering what it is about the aerosol poly that requires such a long time between coats. I've read that the aerosol is thinned with acetone, while the brush on version is thinned with mineral spirits. Is that what causes the difference? I find it amusing that the aerosol is labeled as "Fast Dryiing Polyurethane" but the recoat time is anything but.

This question is asked more out of curiosity than anything else. I've gotten around the problem by thinning the brush on Poly and spraying these small parts using an air brush. Or, I just spray lacquer.

Can anyone shed some light on this?

Ernie
I thin poly with Acetone - & @ 50/50 poly/Acetone I can guarantee its quick drying :)
 

Jeff

Jeff
Corporate Member
A new game show sponsored by Minwax..."Confound the confounded".

I don't know their rationale but I'll take a chemist's stab at an explanation. Dry time refers to the solvents used (acetone, butane, propane, or mineral spirits) not the cure time for the resins to reach their optimum cross-linking and hardness.

1. Proprietary polyurethane resin #1 is in Mineral Spirits, it dries slower and it's fine for brushing, but it has a specific profile for drying/recoating intervals.

2. Proprietary resin #2 is better suited for rattle-can use whereas resin #1 is poor. Resin #2 is formulated in a mixture of acetone, butane, and propane for spray cans and that should make it fast drying, but not necessarily fast curing.

OMG, both labels say "fast-drying"! Why are we confounded?

http://www.minwax.com/wood-products/clear-protective-finishes/interior/minwax-fastdrying-polyurethane
 

ErnieM

Ernie
Corporate Member
A new game show sponsored by Minwax..."Confound the confounded".

I don't know their rationale but I'll take a chemist's stab at an explanation. Dry time refers to the solvents used (acetone, butane, propane, or mineral spirits) not the cure time for the resins to reach their optimum cross-linking and hardness.

1. Proprietary polyurethane resin #1 is in Mineral Spirits, it dries slower and it's fine for brushing, but it has a specific profile for drying/recoating intervals.

2. Proprietary resin #2 is better suited for rattle-can use whereas resin #1 is poor. Resin #2 is formulated in a mixture of acetone, butane, and propane for spray cans and that should make it fast drying, but not necessarily fast curing.

OMG, both labels say "fast-drying"! Why are we confounded?

http://www.minwax.com/wood-products/clear-protective-finishes/interior/minwax-fastdrying-polyurethane

So it appears that both types should cure at approximately the same time - but the aerosol version should dry quicker. However, no finish that I know of requires the prior coat to "cure" before recoating. Besides, I doubt that any poly finish is cured in the 72 hours they recommend to wait before adding a subsequent coat. Why the brush on doesn't wrinkle and the aerosol does still doesn't make any sense to me. In fact, if you brush on a coat of poly over a previous coat that was sprayed with a rattle can, the brush on coat will also wrinkle if you haven't waited at least 72 hours. Since I prefer several coats of finish, sanded between coats, I'm afraid, at my advanced age, that I'll never have enough time to finish a piece. :rotflm:
 

Skymaster

Jack
Senior User
poly, cut 25% min with mineral spirits, gfet a pack of cheaor washcloths at wally or dollar, take brush and toss to other side of shop and leave it there :}:}, quarter wash cloth, make nice pad, dip into poly apply, wait hrish, gently touch dryto touch? lightly sand or nylon scrubbie pad, wipe dust off, apply. wait hrish, repeat till buildup you like.NO DONT PICK THAT BRUSH UP. LOL LOL LOL LOL
 
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