Finishing with polyurethane

[greg.smith]

I'm about to begin application of a single pack PU to finish a coffee table and the manufacturer recommends "lightly sanding" between coats.

How light and with what grit is best? The timber is messmate.

Thanks,
Greg

[Sprog]

I usually go to the manufacturers web site and look at their recommendations.
If I can not find the information I require, an email usually brings the required information.
They want you to get the best result from their product so that you will continue to use it.

[echnidna]

!20 wet and dry. Sand it wet with water or you might get little gummy balls as you sand.

[vsquizz]

I have found using a sealer prior to thinning the first coat (about 5-10%) helps with some brands.

I use 180 or 240 grit W&D between coats depending on how much of a mess I have made.

These days I try not to use the stuff.

Cheers

[pete_d]

Hiya. I've been playing with Poly for a finish on a solidbody guitar, and after much experimenting I've found that those 3M sanding pads work wonders as a wet sanding cutback. I use the 320grit, dipped in water (it's like a sponge) and a tiny bit of liquid soap to act as lubricant.

I've gotten good results so far, but going to experiment with FW Proofseal before doing the actual job.

[Hellacopter 1]

G'day Greg


I use a soft grade steal wool between coats like a 0000 grade or 000 and I find the finish is immaculate. It will take of all the little lumps and won't leave a scratch. Give it a go I'm sure you will love it. Do you spray or are you using a brush??

Hellacopter1

[AlanS]

OOPS...I have been finishing a project, mixing poly 50/50 with spirits, and padding it on. I HAVE NOT sanded between coats...What will be the consequence? The finish is adhering...no bumps or wiskers. I plan to 0000 steel wool as a final step before a nice coat of wax...I HOPE I DIDN'T BOTCH THIS UP! Alan

[greg.smith]

G'day Greg


I use a soft grade steal wool between coats like a 0000 grade or 000 and I find the finish is immaculate. It will take of all the little lumps and won't leave a scratch. Give it a go I'm sure you will love it. Do you spray or are you using a brush??

Hellacopter1

I'm using a brush to apply the finish which has turned out to be blemish free. I did try 240 grit very lightly but given the cool temps here in Melb at the moment it seemed a little too aggressive even though it was allowed to cure for 3 days.

Thanks Hellacopter for the tip, I suspected that steel wool would be best. Thanks to all that responded.

[soundman]

There has been quite a bit of discussion on the board about poly in the past doing a search should yeild quite a bit of info.

but heres a couple of points.
must thin the stuff if you expect a good result. 10% is a good start.
using a matching sanding sealer makes life much easier.
this stuff does take some time to go off. in Brisbane in summer the best I can hope for is one coat every 24 hours, if you try sanding befor its off properly the paper clogs something shocking even wet & dry.
I have hand sanded wet in the past but last few times I used the random orbit sander on low speed with very good results, 240 gritt on the ROS seemed fine between coats, hand sanding you may want to go finer.

cheers

[Wongo]

Yeah 0000 steel wool.

[soundman]

sorry my previous post raved a bit & didn't answer the original question.

In my opinion the words "light sand" are a huge joke. I recon its a marketing phrase to make you think its easier than it is.

The main purpose of sanding between coats with poly is twofold.
1. to flatten out any surface iregularities such as dust, grit & raised grain before the next coat makes the matter worse.
2. to give the following coat something to key into.

if you recoat poly shortly after it "goes off" the following coat will chemicaly key into the previous by partly disolving the surface.
if you recoat poly after it has "gone off properly" the following coat will not key at all unless you sand.
I have seen a couple of jobs where poly has been recoated after it has gone off & without sanding & the top coat has be able to be scratched thru with a fingernail. With a little effort large areas can be scraped away with the fingernail.

so you should sand till it feels smooth, no lumps or bumps and till the whole surface has a matted appearance in that it can be seen to have been sanded.

sand then wipe with a damp cloth & look closely are there any little glossy bits.

It also depends on how keen you want to be. You can get a fair finish with poly quite quickly, but a realy good finish takes a lot of effort.
If you want a realy good finish try something else.
cheers

[Dusty]

Good post, Soundman. Informative.

I agree, the term 'light sand' is misleading and has quite possibly caused more problems than what it's worth.

When cutting back polyurethane don't be afraid to get right into it. I'm always keen to see the surface turn opaque on a decent cut back.

It's really worth putting in the effort to create a great finish. A great, or, on the other hand, an indifferent finish is the first thing non-woodworkers notice about our work. So give 'em nothing to pick at.

[HonkyLips]

I have no idea if this makes a difference or not, but once I have "lightly sanded" the first coat, I wash the boards clean with turps and allow them to dry before applying the second (or third) coat. I have assumed that this removes any bits of grit or dust which may mar the finish.

I've used Cabothane a lot and generally sand the first coat quite heavily with 240/320 as I find it raises the timber grains a lot. I spend more time on getting the second coat even, but will nearly always find brush strokes or drips which I lightly sand back with 320 and apply a final third coat.