Issue sanding between coats of polyurethane

Question

So I am playing with finishes for the first time and experimenting with polyurethane. My problem involves quoting the instructions on the can and giving some background into my process. Bear with me.

From other answers on WW I understand that applying only a few coats of poly can be sufficient but traditionally you could use as many as 21 over the course a year. Since I don't need an exercise in futility I opted for 3 coats.

I did 2, what I thought were, thin coats 3 days apart and the last one was 2 days previous to now. I did not sand between coats; again because it is not required between coats (Again based on other answers required).

Referencing steps from the directions on the can and the Minwax website

4. Apply a THIN coat of Minwax® Fast-Drying Polyurethane using a high-quality natural or foam brush.

5. Let dry 4-6 hours. Then lightly sand entire surface with fine sandpaper (220 grit) to ensure an even finish and proper adhesion. Remove all dust.

6. Apply second coat. If third coat is desired, repeat step 5 before application.

Aside from the lack of sanding between coats I adhered to the instructions rather well.

I left the table to cure in my shed. Issue with that was my shed seems to have become a haven for boxelder bugs. Which would be an issue except they pissed on the table in several places. Some of it just chips off but I needed to sand out several of them. Sanding with 220 grit with only a few passes it looks like I sanded through the two coats down to wood!?

Why would they suggest I sand with that grit if it takes away the coats to begin with? Obviously they didn't intend for that to happen and I am doing something I am not supposed to do. Should I have used a finer grit? Are my poly coats to thin?

Answer 1

Referencing steps from the directions on the can and the Minwax website

Let dry 4-6 hours. Then lightly sand entire surface with fine sandpaper (220 grit) to ensure an even finish and proper adhesion. Remove all dust. [My emphasis]

The highlighted portion is part of the problem we face in the woodworking world: that even manufacturer instructions can't be trusted to be accurate.

The most egregious part of this is that sanding is not needed to ensure adhesion between fresh coats of poly. And actually it's not needed between coats of any varnish and never has been. There are uncountable examples of this to prove the point, as Bob Flexner makes some reference to in The 7 Myths of Polyurethane on Popular Woodworking.

In addition to that, sanding between coats specifically should not be relied upon to ensure a smooth, uniform finish (there are much better methods, none of which waste finish needlessly, more on this below).

It should be done for one reason only: de-nibbing, where it serves a specific, and necessary function for most users.

Note: where de-nibbing is not needed, e.g. where varnish is applied in a clean-air environment, sanding between coats is never done as far as I'm aware.

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From other answers on WW I understand that applying only a few coats of poly can be sufficient but traditionally you could use as many as 21 over the course a year.

I don't know where the confusion arose but the 21 coats refers to a traditional drying-oil finish, not varnish.

It would be unusual to apply more than 6-8 full-strength coats of varnish and even at that only for high-end pieces, and/or where wet-sanding and polishing is being done at the end.

Sanding with 220 grit with only a few passes it looks like I sanded through the two coats down to wood!?

I wouldn't at all be surprised by this, sanding properly — not just scuff-sanding where you're only looking to scratch the surface — will very easily break through only a couple of layers of poly. Particularly cheap poly, which Minwax is an example of.

Why would they suggest I sand with that grit if it takes away the coats to begin with?

This is one of the other reasons I personally object to that part of the finishing instructions. Ignoring that it's not needed for adhesion what it does is pointlessly remove varnish you just applied, only to be replaced by more varnish in the next layer(s)!

Although some sanding between coats is needed for some users, it's actually a very self-serving direction on the part of the manufacturers as they're assuring you use more of the product than needed. This is just like the directions for some stains and for oil/varnish blends, where supposedly only by applying a liberal excess of the product and then wiping almost all of it off can good results be achieved. This is very far from the truth, and it wouldn't be an exaggeration to say it can waste 95% of the product that goes on the wood.

If manufacturers were truly interested in helping their customers to apply the product most effectively, as they say or imply, what they would do is give people specific tips on how to apply smooth, even coats of varnish such that no sanding would normally be required (except to de-nib).

Should I have used a finer grit?

Possibly, but I think you should have first tried a non-sanding approach. Wiping down with warm soapy water would have probably been the first thing I would have tried :-)

Answer 2

"Hot-coating" (applying a second coat of poly over the first without sanding) has become an accepted technique. It seems to work better if done right after the first coat has dried, perhaps because it's still polymerizing. After two coats, common wisdom seems to be to let it cure completely and sand lightly before doing another coat/hot-coat pair (if desired); I admit to being a bit cavalier about that.

When they say "sand lightly," they mean very lightly, just enough to roughen the surface a bit to help the layers mechanically lock into each other and reduce the risk of one peeling awsy from the other (which is supposedly possible). A couple light swipes may be all that's needed, especially -- again -- if the poly is still curing.

[Useful trick while I'm here: After the varnish has really cured, a brown paper bag is just abrasive enough to knock loose dust specks caught in the surface and polish it a bit.]

"Traditionally"... According to whose tradition, with what intent and for what use? You can certainly "build" varnish quite deep, if you want that look, but unless you're finishing a bar top or some other surface expected to get a lot of wear and moisture you shouldn't need it. This depends on the specific varnish -- viscosity varies and some build faster than others -- and how you apply it (padding typically puts down a thinner coat than a well-handled brush) -- but three-to-five coats should give you as much protection as most things will need, and can give you quite a nice shine. This does depend a bit on the wood; some will absorb the first coat completely and only subsequent coats actually stay on the surface...

I put six padded coats of poly on the end table I just refinished and got a "wow, look at that shine" reaction... but the first two were tinted (which I usually avoid on principle, but in this case it was handy) and the last one or two were just cheap insurance.