Polyurethane

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Hardcore Icon wrote:

maybe it's laziness on my part, but I pretty much always sand between coats. it's often easier than inspecting all surfaces visually- a fast swipe of each surface with medium-to-fine-ish sandpaper will tell me by feel how the last coat flowed out, whether there is any crud stuck in it, etc.
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The poly is still setting up in the 48 hour time period, so the next coat will adhere to the previous one quite well. After 48 hours you need to scuff the surface to get decent adhesion between coats. The manufacturer's recommendation has nothing to do with controlling the smoothness of the coating.
And....you will have a LOT better results if you thin the poly in the can down with mineral spirits. I use 2 parts poly and 1 of thinner. Fewer brush marks and runs and bubbles. That message is not on the can.
Downside, because you are applying a thinner finish, you will have to add another coat to get a good build up. But its worth it.
Old Guy

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Hardcore Icon wrote:

Yes.
320 wet-dry on block, wet, with mineral spirits. Takes about 2 minutes to wet sand a 1 x 3 foot bookshelf between coats. Wet paper cuts _fast_, like an eraser. Hardest part is not sanding through.

That many coats, you should thin the poly 50% and wipe it on. Rub out with gray Scotchbrite between coats.
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What 'Pop' says . . . with a couple of provisos . . .
I came into this late, so don't really know if we're talking about 'solvent' or 'water-based' Poly. I'll presume solvent, because of the suggestion to use 'Mineral Spirits'.
When I Varnish trim, 6 coats is usually the minimum. The first coat thinned 50 percent. 'Cure' for 24-hours. {At this point, the surface guides my sanding - which is more dependent on the 'prep work'}. Lightly 'scuff sand' with 220. Wipe with solvent dampened cloth.
Second coat thinned 25 percent. 220 again.
Third coat 'full strength'. Scuff sand with 320. Follow the same process and use finer & finer paper - 400, 600. Let cure 'hard' - at least 72-hours. Lastly, a fine automotive 'compound' and a couple of well-buffed coats of wax.
The 'modern' or 'synthetic' varnish is handled the same.
For 'in-home' use my preference is for the Water-Based Poly. I find the surface to be harder & more abrasion resistant. In addition, there is no 'burned hydrocarbon' smell when I use it {we have gas heat, etc.}. I use the same application 'technique', only do not 'thin', or as much, depending on the 'age' of the can. The H2O stuff flows much more easily. Also, 'dampened with water', vs. 'solvent'.
I don't use the 'wet sand' method, so really have no comment on that.
Regards & Good Luck, Ron Magen Backyard Boatshop

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Ron Magen wrote:

Flooding the paper with mineral spirits keeps it from clogging, making it cut what feels like 10x faster. Those little "pills" that form on the paper hold the paper away from the varnish, preventing it from cutting. The wet paper seems to last forever, too. You can scuff sand an entire bookcase with a single quarter sheet of 320, and the paper still feels new. You could use stearated paper for nearly the same effect except on waterborne.
I prefer mineral spirits because it won't swell the wood if it gets through any breaks in the new finish. Won't hurt the finish provided it's dried hard overnight.
Wet sanding with 1500 or 2000 grit from your Scary-Sharp supplies puts on a nice shine after cure.
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