View Full Version : Sanding sealer to smooth old varnish

01-05-2014, 02:13 PM
I知 refinishing a decked sailing canoe that has Man-O-War satin varnish on the plywood deck. Satin was used for a quick and easy finish. The varnish is in sound condition, but not smooth enough for the high gloss Petit varnish I値l be using this time. I think the easiest way to smooth it would be to use one or two coats of Petit sanding sealer, sanded smooth. I知 wondering if there will be a problem with compatibility of the sanding sealer with the old varnish.

01-05-2014, 02:21 PM
Hmm, seems to me gloss over satin won't be very glossy either way. Why not try a test spot with the sealer? I've always done 4- 6 or more coats of varnish with a good sanding in between.

Jay Greer
01-05-2014, 02:27 PM
Sanding sealer is just what the name implies. It is meant to seal the open grain of the wood in order to allow faster build up of the full strength varnish. In truth, it is often nothing more than thin varnish. If you thin your own first one or two coats of varnish you will be saving money. Satin varnish is intended for interior finish work and has much less weather tolerance than gloss varnish does for exterior work.

David G
01-05-2014, 02:47 PM
The sanding sealer MAY adhere to the old varnish... or not. But that's not why you shouldn't use it. It's formulated, as Jay says, to seal raw wood, and sand easily. It's NOT formulated to be a barrier coat between two products. It will provide a weak link. A potential failure point.

Just sand what you've got until there are few or no glossy bits left, then start varnishing with the new product. You might, as mentioned, thin the first coat or two, then switch to full-strength. Gloss over satin will look gloss.,. mostly. The refractive qualities of the additives in your existing finish will be much subdued by the several layers of new, gloss, product.

01-05-2014, 04:15 PM
Thanks everyone for your help. I guess I'll skip the sanding sealer.

Jay Greer
01-06-2014, 03:38 PM
David is correct. Some sanding sealers contain talc for fast fill. Not good for marine use as varnish does not stick well to it.

Bob Cleek
01-06-2014, 04:41 PM
What Jay and David said... You can sand the satin finish well without going to bare wood and then build up your gloss varnish full strength on top of that. You can simply scuff it up well with a Scotch Brite abrasive pad (the burgundy colored ones available at the paint stores) between coats until you have three or four coats built up. Let that dry well and then sand it smooth and then continue building up coats until you have eight or ten down. You will find that the gloss varnish will fill well. What you don't want to do after you initially get it smooth is to sand it down so far that you are taking off the varnish you just laid down. It is a very common mistake to "over sand" between varnish coats. People will spend great amounts of time putting down eight or ten coats of varnish, but sand so much between coats that in the end they only really have about two or three coats worth left!

01-06-2014, 06:22 PM
There are glossy finishes and there is shiny varnish. How yours will turn out depends a lot on the surface you start with. If it is at all rough, then you will end up with a shiny coat of varnish over a rough surface. Gloss comes from the combination of shiny varnish and a smooth base surface. Basically the varnish is shiny, but it coats the surface pretty much exactly, so any imperfections will show up as optical irregularities (the reflected light will get reflected differently by different parts of the surface. Satin varnish is the same exact thing, only at the microscopic level (the put minerals in the varnish to scatter the light, so it doesn't reflect like a mirror).

So, from my experience, the first steps toward a really glossy finish is getting the surface you are going to varnish as smooth as you can get it...and THEN start varnishing and sanding.