Working on a paint job on my Formica counter top (primer, granite spray paint, and polyurethane finish). This is the first time I've used Polyurethane and I'm getting different directions from different sources.
My question for anyone who would know:
Do I need to sand between coats of Poly?
I'm going for 4-6 coats, have the first coat on and dry, and am not sure if I should sand or not. The directions on the can say "recoat in 6 hours. If first coat is allowed to dry 48 hours, sanding is necessary between coats."
Does this mean if I don't wait 48 hours that I do not need to sand?
Generally speaking do what the coating manufacturer says, not what some self-proclaimed expert says unless it's more conservative.
In the extant case, if you recoat in less than 48 hours you don't need to sand. I would anyway if I were going for a good finish--the smoother the surface the better the finish.
Is there not the concern that the finish has to completely harden before sanding? I would think (w/ nothing to back it up) the longer the better. For the sake of the after-sand finish quantity/quality. Like multiply x 5 to be safe. Fives a good average number for things like that in general, any opinions? Quit asking questions, you're confusing us.
I have read in here that (i)only sand is to worsen the finish, OR (ii)work in a polish regime,which may include some form of sanding to improve the finish. If you can recoat before the dry time w/o sanding, and you're not gonna super-finish, thats what I'd do.
Maybe I'll hit it just a little bit with some 220, it hasn't been 48 hours but if it will make a better finish then that's what I'll do. But the directions only said to sand if it's passed 48 hours so going by that I guess I don't *have* to sand.
That is correct; if you recoat quickly the new coat can bond. If you wait you have to rough the surface up a bit. I like to put on a few coats, sand (well, steel wool) and then put on my final coat. The more often you sand, the better chance you have of getting a smooth final surface. But it is work and makes for a thinner coating.
Reason to sand: Air is not dust free; can see dust particles. Reason not to sand: In specs per the directions and surface looks perfectly fine, no dust, bubbles or other contminants visible.
Since this is not wood (it's a countertop that is painted to look like granite), whatever dust particles can't really be seen anyway. Good reason not to sand.
The main goal here is to get a real good thick protective coat. Just put the second coat on a few hours ago.
maybe it's laziness on my part, but I pretty much always sand between coats. it's often easier than inspecting all surfaces visually- a fast swipe of each surface with medium-to-fine-ish sandpaper will tell me by feel how the last coat flowed out, whether there is any crud stuck in it, etc.
If the finish has to completely harden before sanding then I'd start using a different product. The precatalyzed lacquer I've been using lately is ready for sanding in 40 minutes (per the manufacturer) but not completely cured for several days.
The main issue with sanding is whether it is hard enough to sand without prematurely clogging the sandpaper or otherwise misbehaving. I can't see where waiting a week between coats is going to gain anything.
The poly is still setting up in the 48 hour time period, so the next coat will adhere to the previous one quite well. After 48 hours you need to scuff the surface to get decent adhesion between coats. The manufacturer's recommendation has nothing to do with controlling the smoothness of the coating.
And....you will have a LOT better results if you thin the poly in the can down with mineral spirits. I use 2 parts poly and 1 of thinner. Fewer brush marks and runs and bubbles. That message is not on the can.
Downside, because you are applying a thinner finish, you will have to add another coat to get a good build up. But its worth it.
yes, for a mechanical bond also after mfr rec'd time passed. My experience is if the wood is "rough" to begin with it may help between coats (specifically after spit coat), but that wood may not be worth improving. If its nice in the first place, sanding after the second coat and after gives little if any improvement for the amount of effort req'd.
320 wet-dry on block, wet, with mineral spirits. Takes about
2 minutes to wet sand a 1 x 3 foot bookshelf between coats. Wet paper cuts _fast_, like an eraser. Hardest part is not sanding through.
That many coats, you should thin the poly 50% and wipe it on. Rub out with gray Scotchbrite between coats.
Try it by the manufacturer's recommendation, but don't be surprised if the second coat drags if you don't scruff between coats. Poly does not bite into the previous coat, it only lays on top, so the smoother the underlying surface, the better. I would not use 220 though. I'd just knock it down with steel wool or 3M pads. All you need to do is break the glaze for a little tooth.
What 'Pop' says . . . with a couple of provisos . . .
I came into this late, so don't really know if we're talking about 'solvent' or 'water-based' Poly. I'll presume solvent, because of the suggestion to use 'Mineral Spirits'.
When I Varnish trim, 6 coats is usually the minimum. The first coat thinned
50 percent. 'Cure' for 24-hours. {At this point, the surface guides my sanding - which is more dependent on the 'prep work'}. Lightly 'scuff sand' with 220. Wipe with solvent dampened cloth.
Second coat thinned 25 percent. 220 again.
Third coat 'full strength'. Scuff sand with 320. Follow the same process and use finer & finer paper - 400, 600. Let cure 'hard' - at least 72-hours. Lastly, a fine automotive 'compound' and a couple of well-buffed coats of wax.
The 'modern' or 'synthetic' varnish {Poly} is handled the same.
For 'in-home' use my preference is for the Water-Based Poly. I find the surface to be harder & more abrasion resistant. In addition, there is no 'burned hydrocarbon' smell when I use it {we have gas heat, etc.}. I use the same application 'technique', only do not 'thin', or as much, depending on the 'age' of the can. The H2O stuff flows much more easily. Also, 'dampened with water', vs. 'solvent'.
I don't use the 'wet sand' method, so really have no comment on that.
Regards & Good Luck, R> > Working on a paint job on my Formica counter top (primer, granite spray
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