Paste wood filler is used to fill up the pores is in open pored woods like oak to obtain a smooth finish.
If you want a smooth finish on an open pored wood you can use paste filler, French polish (the pumice used as part of the first coats fills the pores), or apply many coats of finish and cut them back until smooth.
Filler is silica (powdered quartz) in a binder. It is thinned with naphtha to about heavy cream consistency and then painted on the wood. As soon as the filer switches from glossy to dull the excess is removed. I use plastic auto body scrapers (perpendicular to the grain) and then switch to burlap. A very light sanding gets the surface ready for the next finish coat.
Behlans still makes paste filler, and it can be tinted to create different finishing effects.

You also want to consider the material you are working with as to whether you need a pre-conditioner or not. For example, I used a birch plywood. Birch has a very tight grain, and using a conditioner will not allow a water based stain to penetrate very well. I think that is one situation in which you would not want to use a conditioner first. For wood with a less tight grain, a conditioner may allow the wood to take a stain more evenly.

I have a red oak. Would you recommend the conditioner and filler? I want to make sure I get a very even stain.

Filler is for evening out the actual surface of the wood. Red oak is not prone to blotching and a conditioner is not typically required.

Sorry to resurrect this old thread but it was one of the closest to what I needed to ask, when I searched the forum for 'finishing red oak.' I have two unfinished red oak nightstands made in a woodworking class. Though they have some beginner boo-boos in the things like dovetail joints, they are overall well made and solid, and I'd like to either use them or give them as a gift, but only if finished. Would a 'sanding sealer' be an acceptable product to fill the pores or do I need to get the silica type filler brickeye mentioned? Any tips on things like doing this in certain type weather, like avoiding heat or cold or humidity? I tried to find a link to Synseal by Dunn Edwards but all I got was the hazardous material info. A less toxic product would be preferable. Thanks in advance.

It is very difficult to fill the grain of oak and get a smooth surface. Best idea is to just stain and use a water based varnish.

"It is very difficult to fill the grain of oak and get a smooth surface. Best idea is to just stain and use a water based varnish."

It is done all the time with paste filler.

Look up pore-o-pak.

You will never get a smooth finish on oak without paste wood filler.
It is one of the reasons so much oak furniture looks cheap, with the pores of the grain still visible in the finish as shallow depressions, especially were a pore has been cut at a shallow angle to the surface of the wood.

Paste wood filler can be applied before or after color, and can be the same color as the wood or contrast depending on the final effect desired.

Ok, point conceeded.

I usually use a wood with a grain applicable to the desired finish---and see no reason to use oak/ash when a smooth surface is desired.

I had not heard of that product---and have not seen it on a woodworking website with over 30,000 members. Was shocked Woodcraft carries it---I spend a lot of finishing money there.

Learned something today. Thanks.

Well, I asked on the other forum. Most pro's prefer Constantines Paste Wood Filler over Pore-o-pack. Have yet to hear from hobbiests.

They also say neither is a good DIY product as they need to be applied (sometimes several coats) sealed with a shellac or similar sealer, and then finished. Seems to be a very involved process.

Ok, a couple hobbiests have chimed in with their experiences---along with several more pros.

All say Pore-o-pak works well as long as you apply it correctly. The application is difficult but not impossible for a DIYer.

I did my white oak kit cabs with pore-o-pac. It's an ordeal. It lessens by 60% the absorption of stain. If you wanted to, you could stain first, then fill, with the filler tinted to match (or compliment) the stain, then maybe a final toning with stain to redo areas where the stain was worn off; the filler is fine-ground silica, and somewhat abrasive.
For me, filled oak is the cat's whiskers. And there's no annoying wild-a$$ oak stain/grain to look at, the finish is very uniform.
Casey

Thanks for the info, I'll look for those two products and see what I want to do. Yes, I agree, oak that has visible 3-D pores in the finish looks cheaply done. I have an old white oak twin bed frame and headboard that my dad made in the 1950s that is smooth as a baby's behind. But he passed away some time ago so he's not around to ask how he did it, or if it was really porous before he finished it! Heck even the slats that hold the box springs up are high quality.

A single application of pore-o-pac is all that is needed.

It is to thick out of the can to apply, but can be thinned with naphtha to the consistency of heavy cream.
Paint it on heavy, and wait for the wet gloss to flash off.
I often use plastic scrappers for auto body work to remove the excess on the first pass.
You remove it across the grain, then smooth it with the grain to make sure everything is filled, then across the grain with burlap.
Shellac may be needed as a sealer depending on the effect desired and any tinting added to the filler.
It does add another step to finishing, but you can choose to emphasize or de-emphasize the pores depending on the color chosen for the stain and filler.
I have about 8 feet of shelf space filled with finishing books, and another 3 feet of notebooks with data on materials and finishes I have used over the years.

I want to maintain the orginal dark stain of the trim in this house. It has a lot of paint slopped on the sides but the faces are in pretty good shape. Firstly, I want to carefully remove the paint and secondly, restain these areas. Also, replaced a built-in with an archway with pilars and header. The wood used for this trim was the original header face, new hemlock casement and stripped original fir for the pilars. I need a stain formula which will bring the newer wood to match the original wood in the room and the header. Any ideas out there?