What is the point of sanding between coats of paint?
Smoothing the surface, so the next coat will be more even
or
Roughing the surface, so the next coat will adhere better
Janet
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Replies
Both, actually.
Sanding will remove large defects (drips, sags, dust), while at the same time "roughing" the surface for better adhesion. The latter effect is most important with hard, high gloss paints like enamel.
It's important to use the right grade of sandpaper. Too fine will take forever if the previous coat has many major defects, too coarse will leave sanding marks that require additional sanding with finer paper and maybe additional coats of paint. You really want to just scuff the surface enough to remove the shine.
Very helpful, thank you!
Janet
Sanding a prior coat of paint is not needed unless you are covering very old, fully dried paint. As usual, its best to simply follow the manufacturers instructions. However, if the surface is rough, then sand prior to painting any coat. No amount of properly applied paint will cover dust nibs, dog hair, or drips without first sanding them smooth.
Dog hair
sapwood wrote:
No amount of properly applied paint will cover dust nibs, dog hair, or drips without first sanding them smooth.
LOL. In my house, it's cat fur that gets into everything—but sometimes the cats do, too.
Once I was roller-painting a wall while listening to music through earplugs. When I bent down to reload the roller I saw, next to the paint tray, PAW PRINTS in paint.
Fortunately, the cat chose to leap onto a piece of furniture that was easily cleaned off. But I'll never forget that sinking feeling...
Janet
Usually you do that kind of sanding when you are going for a shiny finish. It takes out minor imperfections and creates a good surface for the next coat. 600 grit wet paper is not unusual for guys painting cars but I use 200-400 most of the time.
Primer
When you are going for a shiny finish, do you sand the primer?
I applied primer but can still see the color differences from the wood grain and some repairs. Does this mean the primer is too thin? I think it is thick enough to sand without reaching bare wood.
if the primer coat is too thin, should I apply an extra layer of primer (as opposed to an extra layer of paint)?
Janet
I'm no painter.
Sand, prime, sand again-to smooth the raised grain from priming (more so w/latex).
You should be able to apply two top coats w/o sanding, but hitting it w/ higher grit again won't hurt neither.
It's been more than 7 years, just wondering if you've forgotten that sinking feeling yet?
Manufacturer's instructions were ambiguous
sapwood wrote:
As usual, its best to simply follow the manufacturers instructions.
That's what got me wondering about sanding between coats. This is my first time using Benjamin Moore Advance (waterborne alkyd) and the labels said
Since it didn't say "sanding OR painting," it sounded as though sanding was always necessary. But did the entire surface need sanding, or just any flaws that became apparent?
Janet
Yes.
Primer only needs to be a thin coat - it's just increasing adhesion between wood and paint. If it's being used to hide underlying color that's a different type of primer and it goes on thick enough to hide whatever you're trying to hide but that extra thickness is only for hiding and doesn't affect the paint/wood bond, which would have been fine with only a thin coat.
If a wall/cabinet/trim has to be very smooth it's normally primed with what's called a high-build primer. It's extra thick so it can be sanded perfectly smooth. Then 100% acrylic paint is used (I like Kelly Moore) which sands very easily unlike most water based paint. Sanding inbetween coats isn't needed for a good bond - it's just to keep the surface smooth.
A more advanced technique to get the absolute best finish is to sand between coats and actually sand the final color coat - and top coat it with a clear acrlic.
With oil based paints there is normally a certain amount of time between coats in which sanding isn't required (say 18 hrs), after which it is sanded with fine sandpaper so the next coat can properly bond. All this type of information is available from the paint manufacturer.
Sanding difficulties
You said that most water-based paint doesn't sand easily. I'm using Benjamin Moore Advance primer, which is a water-based alkyd.
When I used a random-orbit sander, the particles formed clumps that stuck to the primed surface and wouldn't come off with a brush or air compressor. A scraper worked, but I decided to give the next piece a lot more time before sanding.
After a week of drying (although it's been very humid), the particles still formed clumps, but at least they stuck to the sanding disk rather than the primed surface. Stuck very firmly, too—the sanding disk didn't work for long.
Is this what you meant about sanding water-based paint?
Is it better to sand by hand?
Janet
Is it better to sand by hand? YES
I would use a sanding block, and a type of sandpaper suitable for sanding fresh paint, and use very easy strokes. I like to use stearated sandpaper, because it has a built in lubricant. Some say don't use it because the lubricant can cause problems with subsequent topcoats. Another choice is Silicon Carbide wet and dry paper. You can use some water with a little dishsoap as a lubricant, for great results. Then there is aluminum oxide paper, which I don't use for sanding new paint, and never have. I use it a lot for sanding old paint. Garnet paper is available, but it's pretty much seen its day, skip that one, and skip flint paper too.
Types of sandpaper
MarkH wrote:
aluminum oxide paper, which I don't use for sanding new paint, and never have. I use it a lot for sanding old paint. Garnet paper is available, but it's pretty much seen its day, skip that one, and skip flint paper too.
Must confess that I never considered the composition of the paper. I pulled the sanding disk off a hanger of unpackaged singles at the paint store. They were labeled with the grit size but nothing else—no brand name, no composition.
Why shouldn't aluminum oxide be used for new paint, and what is wrong with flint paper (which I've never heard of) or garnet paper?
Janet
My recollection is that flint lasts about 30 seconds -- the grit starts falling off as soon as you take it out of the package.
I never have used aluminum oxide for sanding between coats, so it could work out for you, I dunno. Garnet paper is outdated but has one advantage over all other papers, final sanding wood before staining. What Dan said about flint paper, yup. Stearated paper is Silicon Carbide, as is wet and dry paper. Wet and dry can be used either way, but works best wet. Stearated paper works better than wet and dry paper on dry surfaces.
Power sanding creates heat and is too aggressive normally. You may have been melting the primer from friction and that caused the balls of paint on the paper. Most people probably don't sand between coats, but those that do mostly use hand sanding methods, or special equipment.
How about sanding sponges?
Can sanding sponges be used for this purpose, or are they not as good as using a block sander?
MarkH wrote:
Power sanding creates heat.... You may have been melting the primer from friction and that caused the balls of paint on the paper.
Clever of you to figure out the problem! So, it's back to sanding by hand, which is better for me, anyway. The vibrations from power tools cause numbness in my hands; then I have to wear wrist splints for a week.
Janet
Sanding sponges work, but I'm not a big fan of them.
Hand Sand!
After priming, it depends on the primer, but I hand sand usually with 180-grit and generate a fair amount of fine dust. Sometimes I hold the paper in my hand, sometimes I use a sanding block. But the surface ends up pretty darn slicky smooth. It can be "blotchy" in terms of there might be more primer left over here, and a little less left over there. As long as you haven;t sanded to bare wood all is well.
I brush off or vaccum the surface, then tack cloth.
If proper prep work was done to this point, the rest should be pretty easy. After the first top coat, it's more of a very light scuff sand than a full-blown sanding session. Just lightly running the paper over the surface to knock down dust nibs, etc. I sand with one hand and run my other hand over the surface, feeling for smooth.
Clean, then apply the second top coat, and you're done.
Yo Mongo
You do it the way I've had success and I'm no painter.
in addition,
There are 11 kinds of people in this world-those binary folks and then that 11th that goes to a fest of not, even tho it's in Connecticutt-the state where that guy lives
no?
uhhhhh, I think there are 10 kinds of people, but WADDA I KNOW?
uhhhhh
I think maybe there's 12.
Those binary guys and then you and Mongo.
Festgoers that ain't said they're goin' this year.
no?
Ah-ha!
Your post had me wondering, so I looked around and found the fest thread. It's now on the calendar. We're hosting visitors from out-of-state coming in that weekend, but I might be persuaded to slip away for a day. They're my wife's friends after all.
Sorry I missed you and the other kids at MikeFest Part Deux. August is always a brutal month for me regarding schedules due to long-standing and pre-scheduled family get-togethers.
How 'bout the Sox and the Tribe? What a difference a year makes, eh?
Well, here's hoping you can break away for the day.........
I mean, so close................be a shame not to.
And yes, but of course..............talking about it might jinx at least our end of the standings, and I've learned long ago. Maybe there's a difference and maybe not.
We started the June swoon early-like last yr almost to the hour-same record b/4 and after-only this time the Tigers have treaded water and we caught up. This coming weekend and monday might do much to tell the tale for later in the season-we meet up with the motor city kitties in a 4 gamer. But first, need to meet up with KC in their park. Never an easy match.
And your former boy-Masterson is throwing some good games-he recently "blacked out" but came back the next outing and recorded another shutout-3rd of the year. If he maintains that class, and the rest do their part-who knows................
Can't say it's very upsetting to see the Yanks faltering and relying on the injury report for a reason. The Sox early in the year had guys in the lineup I had never heard of. Man, it was like a new team. They made it through that well enough. Depth at a reasonable price is nothing to sneeze at.
So, yeah-show up and we'll talk baseball.
Joyce says hey.
12 ain't binary, now I don't know what either of us are talking about. Unless, you're talking about decimal bitheads not going. I can't make it this year to the fest, would like to, but things aren't the same anymore.
Mark
yup, decimal points and not showing up. Too bad, always a blast to get together with those ficticious names on the board-or in this case-across the boards.
Don't know how I should act................
Guess I'll just be myself, that aught to throw 'em.
Oh, just be yourself, it'll all be OK!
Personally I'd trash your primer and get something designed to be sanded. All the talk about how to sand a gummy primer has my head spinning - it's not difficult - simply use the correct product. At this point you are better off topping the primer you have with a 100% acrylic paint that's sandable. Kelly Moore durapoxy sands well by hand or sander....it only takes 320 grit.