Sanding Between Coats

[bpolvo]

I am refinishing old mahogany that has been darkend by years outside. What I'm concerned about is the sanding between coats of varnish. I prepared the wood by striping any old varnish and stain off, sanded and cleaned the dust off. I varnished the board with Epifanes Spar varnish. After it dried, I used a 400 grit dry sandpaper to rough the surface for another coating of varnish. When I do this the board and varish turns a 'white' color from scraping the varnish. Is this normal? What can I use that won't turn the varnish? Will it disappear after the next coat of varnish? What causes this?

Thanks, Bob

[markharrison]

Completely normal and wont be seen after re-coating. Just be sure to wipe off as much of the dust as you can. I just use a rag with metho.

[Dean]

you get the same effect when sanding poly-u too

[bpolvo]

Thanks, I feel a lot better. Didn't want to ruin the wood or finish.

Bob

[Eastie]

It is normal, but be aware that the 400 you are using may be digging through the varnish surface and leaving deep marks that show through subsequnet coats. If this shows to be a problem change to 600 or 800 (or even 1000) between coats. The sanding between coats helps remove high spots and even the varnish surface, you don't want to cut through previous layers of varnish.

[markharrison]

What Eastie is referring to are called "witness marks". Basically these are where you rub through a coat in one spot. This is more particularly a problem with poly and other varnishes. Shellac and lacquer less so as these finishes dissolve into each other.

Good advice though. You don't need to be aggressive to get the right effect unless you are trying to rub out a run/sag/etc. 400 should be okay most of the time but as Eastie says, you may find it easier with finer grit paper at times.

[surfdabbler]

Also, make sure you use fresh sandpaper, and when it loses it's sharpness, get a new piece. I've had the problem before when filling the grain (where you sand back to the wood), and in sanding back to the wood in parts, old sandpaper burnished the surface of the wood where it cut through the varnish. The burnished sections didn't take the later coats of finish properly, and showed up in the final product as spots of slightly different colour.

With most PU finishes, the white powder doesn't matter too much, but I've seen it with nitro lacquer where the white powder affected the later coats, and the final finish after wet-dry polishing was spotty white. But, a quick rub of a matching stain followed by a soft wax polish got rid of the white spots, and it looked great after that.

Anway, I always wipe off the white powder with a damp rag now before recoating. With varnishes, it doesn't matter too much, but it's a good habit.