Sandpaper or steel wool in-between coats of polyurethane?

[Jason White]

Using Minwax wipe-on poly (oil) for a table top. Been sanding with 400 grit in-between coats, but it seems too aggressive -- especially with my random orbit sander.

Is 0000 steel wool a better way to go in-between coats? Will it abrade the surface enough for the next coat of poly to stick?

Jason

[Cary Falk]

The problem is not with the 400 grit sandpaper. The problem is using a random orbital sander. I use 400 to 600 grit sandpaper for finishing but do it by hand. I never liked steel wool.

[scott vroom]

What Cary said.

[Jim Mackell]

I use 0000 steel wool by hand and vacuum the surface thoroughly afterwards. Or Scotchbrite pads. Whichever is handier. The random orbit is way too aggressive, regardless of grit.

[Doug Hepler]

What Cary and Scott said. Steel wool is OK for the final coat but it tends to leave boogers while sandpaper tends to flatten them or knock them loose. I suggest that you use a cork block or a wood block padded with fabric.

[Prashun Patel]

only sand in between every third or fourth coat. Use 400 or 600 by hand. Sand to knock down nibs, not for adhesion. Adhesion has never been a problem with mineax poly for me.

[Steve Schoene]

With poly you really should sand between each coat unless there is a specific "no sand" time window specified in writing from the manufacturer. But I agree that random orbital sanders are too aggressive. By hand use 400 grit on wiped on finish or 320 with brushed on varnish. You can avoid some sanding if you wipe on the varnish in "sets" of three coats with only enough time between coats to get the finish barely set. Then after each set let dry overnight and do sand before the next set.

[Art Mann]

I spray a lot of polyurethane or sometimes lacquer on the products I sell and I stopped sanding after each coat a long time ago. There is a maximum time specified on the container that allows for recoating without fully curing. If you stay under that time, you can recoat often. I typically do 5 coats and only sand lightly with 400 after the third one and sometimes the fourth. The result is just the same or better than if I sanded between every coat. After the final coat, I like to allow 48 hours under ideal curing conditions and then "sand" the piece a final time with a section of brown paper bag to remove the nibs that showed up after the final coat. I do this on carved coasters and trivets that take the worst kind of water, heat and abrasion abuse and the appearance and durability are better than I expected when I got started. Don't assume you need to sand between coats until you have proven that it is necessary for your particular application.

I never use steel wool anymore because I don't like dealing with the iron particles.

[Wayne Lomman]

Forget the steel wool. It creates more problems than it fixes. Hand sand with 320-400 paper as often as you need to for nib removal and flattening. Machine sanding that kind of poly is not a good idea. Work with the curing tables and recoating times recommended by the manufacturer. Cheers

[Prashun Patel]

Steve, he is using wipe on poly. Do you recommend sanding in between wipe on coats? I have never done that and never had problems. When does this become an issue and what does it look like when it fails?

[Steve Schoene]

Poly has more bonding problems than non-poly varnish such as Waterlox. If it has dried a day or two I would definately sand, albeit lightly. If it is only 3 oe 4 coats between coats sanding isn't needed and in fact may be almost impossible with the varnish so little cured. Failure with poly may well not happen immediately, but down the road perhaps when someone bangs the surface with a toy and a spot flakes off, when if fully bonded a small dent may occur. One might never attribute the damage to poly failure, but in my mind it's better to be safe than sorry.

I mentioned the application of wipe on varnish in "sets", but I should add that I find this much preferable to allowing full drying time between each wipe on coats. One reason is that the bunched coats link together as they cure so that they can be sanded and rubbed out without revealing witness lines. Witness lines before the last set don't matter since they disappear when the next coat is applied.

[Scott Holmes]

I say never use steel wool between coats.

Reason #1 you will leave shards of steel wool behind; then you will top coat it. Now you have steel wool IN your finish when water vapor get to it, and it will, it will RUST - IN YOUR FINISH.

Reason #2 Many steel wool products have oil to keep them from rusting so now you have oil on your surface... bad idea.

Reason #3 Poly does not stick well to itself or anything else for that matter that is why they tell you to sand between coats. Steel wool abrades the finish; it does not give enough "tooth" for poly to adhere well. 220 sand paper 320 at the finest.