Filling grain with sanding sealer

Sanding sealer instead of grain filler - is this an acceptable replacement? October 16, 2003

How safe is it to fill in the grain of porous woods like mahogany with many coats of sanding sealer instead of using the usual grain filler?

Forum Responses
(Finishing Forum)
I don't think it's a good idea if you're trying for a smooth finish. You'll have to put about 3 or 4 thick coats on, then sand all the finish off the high spots to bring it down closer to the grain level. Then you have to start finishing again. Depending on the type of finish, you could get discolor or other problems over time doing this. I think it would be faster to just fill the grain in the first place and take much less material.

Also, sealer is the weakest link of the finish system. You are better to build with high gloss. Just be sure to not go over any dry mil thickness limits.

Bob Niemeyer, forum technical advisor

We have had great luck filling open-pored wood like mahogany, walnut and oak with bi-component polyurethane sealer. There is no problem with excessive coats like there is with conversion varnish. We do all our filling with the sealer, then topcoat. I was told that the polys have a sanding resin unlike the American products with sterate. Is this correct?

2k urethane sealers are made to build unlike the US products mostly used, i.e. vinyl sealer, NC sanding sealers, precat sealers, etc. I feel that 2k urethanes do sand better than most other sealers. People need to remember that European finish products (PE and PU) are used completely different than conversion varnish or precats or NC. Once US finishers see how much better they are for kitchens and arch. finishing, they won't go back to the old ways of doing things. I am not saying it's the cure all for finishing, but it does have its place.

Bob Niemeyer, forum technical advisor

I have to agree - 2k polys sand like a dream. If anybody remembers sanding the old deft lacquer, it's about the same. I've even seen the material dust up when it's still wet (but wait for it to dry). Isn't the resin different to allow for this easy sand but to still keep the integrity of system?

That is why you see European finishes having separate sealers and topcoats and very few "self seal systems" like what we see more of here in the US. Each product is built to perform a certain function, whereas selfseals have to bridge the gap of fulfilling the needs of both products and will suffer in performance somewhere. That comment gonna stir the pot, huh?

Bob Niemeyer, forum technical advisor

First of all, you do not want to fill the grain with sealer alone. Many products advise otherwise. Use only one full wet coat of sealer and the rest is topcoat. The secret is in proper sanding between coats.

From contributor D:
Polyester filler is the nuts followed by acrylic urethane topcoats. You can't beat this system. Learning to hose on polyester is a challenge and waiting overnight for it to dry sort of sucks and trying to get it out of the gun in 10 minutes so that it doesn't turn into a hockey puck can be a challenge in 115F temperatures, but other than these minor details, the stuff rocks.

In using the poly filler, does it have the same pot life as the poly topcoat and problems as you mentioned?

There are still many shops around the country that fill the open grains with paste wood fillers. And not every finishing shop is using catalyzed coatings, and many never will. What works for you is not always the right coating for someone else.

Sealers were never intended to build a finish.

Those who do use component coatings should always wear their protective gear, and have the proper ventilation in their shops.

From contributor J:
Why paste wood fill and stand around waiting for it to dry when you can get full fill with PE or PU with only an extra coat or two? Sure, plenty of shops will never go to high-tech coatings. Or maybe it's because they are ignorant to try them. Like they say, can't teach an old dog a new trick.

Lots of smart guys have gone over to the 2k coatings and lost their shirts. It's not for everyone.

Isn't finishing more than just about filling the grain with a one extra coat?

One reason that I use a paste wood filler is that sometimes I want to put a different color into the pores of the wood and not just fill them with a clear coat. I finished quite a few matched rift oak panels with a green dye stain and a yellow wood filler. On another job for a bank, I used a black dye stain on 15 4X8 ribbon striped mahogany panels and then once I sealed them I applied aluminum powders melted into carnuba wax into the pores. Sure it took me awhile to complete them, but my bid allowed this, based on my sample time, and the architect's decision to go with it.

From contributor D:
The 2K urethanes do have a pot life, but it's in hours, not minutes, which is the case with the polyesters. You can easily deal with a four hour pot life. 10 minutes is a whole other story. With polyester, you really have to move.

Why go to the expense of acrylic over the polyester?

From contributor D:
The urethane is available in multiple sheens; the polyester is super high gloss only. Also, the price of polyester or acrylic urethane topcoats are very similar.

From contributor J:

You guys are taking something and running with it. I still glaze to color pore and do many other items that have never been talked about on this forum. No, urethane isn't the cure-all, but it does have its place in the finishing room. It's a time thing - glaze can dry in 15 minutes. Most fillers need overnight or as least several hours. An extra coat of PU? Only 45 minutes.

Why not add mineral spirits to the paste filler to make it thin and wipe it on to fill some of the pores? Anything you do to fill the pores will help in saving time building up and knocking down whatever you decide to use. Especially if you have raised panels and mouldings and can't really knock your buildup down with a machine. And remember, whatever you use, your first coat must be a piss coat so you don't trap air, especially in mahogany. Industrial Finishing Products in Brooklyn has a product called car-seal. A nice first coat that fills real nice.

Why not use alkyd based oil wood filler? Just add a little oil color to enhance the grain, wipe it evenly and dry overnight. Then apply stain or sealer directly, then tone to even the color, then topcoat.