sanding prior to painting

I need to finish a LOT of house trim. It has been milled and sanded to 220.
I have been thinning and spraying Benjamin Moore's "Regal" undercoat a nd "Impervo" top coat. These are water borne and need some care in sanding . My first thought was to raise the grain with water, sand to 320, prime i t with the undercoat, sand again with 320, and finally 3 coats of the Im pervo.
Since I am getting into "mass production" now, I would like to sreamli ne the process a bit. Since I am using water borne paint, it will raise th e grain. Can I just spray the undercote, and sand to 320, and then apply t he top coat?
Len
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Sanding to 320 is way overkill IMO. It is already at 220 which is more than enough for primer. I'd spray the primer, sand again - lightly - with 220 and do the top coats. If this is outside trim I wouldn't bother sanding the primer. If it is inside trim I might sand lightly but I wouldn't be using waterbase Impervo, I'd be using the alkyd Impervo.
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dadiOH
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On 6/9/14, 3:32 PM, Len wrote:

You may want to consider Zinsser B-I-N primer. It's pretty expensive because it's shellac based, but it's well worth it. It'll save you on sanding because it dries so quickly and doesn't raise the grain like water. It also sands baby-butt smooth, so you'll likely save a top coat or two.
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I agree with dadiOH and haven't sanded paint finishes past 220 in years. I f you are sanding correctly, you simply don't serve any purpose in doing so unless that suits your personal taste. I would say that 99% of people san d way too much anyway to make up for woodworking deficiencies, poor finishi ng, and for some reason it seems to make some folks feel like they are conn ecting to "fine woodworking" when they get to the sanding stage.
As far as sanding latex, I don't know any professionals that sand latex unl ess they are correcting the incidents listed above. It isn't necessary, an d I have even gone weeks between coats and had no adhesion problems. Latex by nature is a porous finish and subsequent coats will adhere just fine. S tay with the same product and there won't be problems.
I like the idea of changing to alkyd finishes. If I can, I apply those.
If I have to prime, I like BIN over all other products. No thinning needed , you can put it on whisper thin, shoot it with the same tip (I use 1.2mm) that you use to shoot finish, and it doesn't foul the guns. Plus... it ju st works.
But Len, if you are in "production mode", why are you priming? Most of the time it adds nothing to the finish if you are starting with new moldings, trims, doors, shelving, plywood, etc. Priming is a good idea if you are pa inting dirty wood, over old finishes, or painting over contaminated finishe s. It is also a good idea if you are painting over finishes in areas that have had a lot of cleaning such as in kitchens, baths, etc., where spray cl eaners and soapy products or other chemicals might leech into your new fini sh.
If you are coating new materials, use the same paint you are using for fini sh as the primer, mid and top coats. NOTHING sticks to a finish like itsel f as it is specifically formulated by the manufacturer to do just that.
Put the sandpaper away, ditch the primer, and keep the Impervo. That is so me really good stuff. I have only used it on one job but shot a lot of it, and it was like shooting liquid plastic. Shot clean, required little thin ning in my pressure sprayer but none in my airless, and the finish tightene d up to a glassy smooth finish when applied at 3 mil for the first two coat s, then a bit thinner on the last.
I think in "production mode" you should load your gun and start shooting.
Robert
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On Monday, June 9, 2014 3:32:25 PM UTC-5, Len wrote:

ng. My first thought was to raise the grain with water, sand to 320, prime it with the undercoat, sand again with 320, and finally 3 coats of the Impervo.

the grain. Can I just spray the undercote, and sand to 320, and then apply the top coat?

I was under the impression that water borne products needed to be sanded to 320 to get a smooth final finish.
Impervo is not latex. It is an acrylic.
I found that the primer/sanding sequence sealed the wood (poplar) and gave me a smooth surface to start spraying with top coat.
I am using a 4 stage Fuji gun. In the earlier tests I found that thinning was needed. But, I can try without thinning again. I don't relish the ide a of 4 coats (primer plus three top coats).
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All acrylic paints are latex; nor all latex paints are acrylic.
None of them sand worth a damn...it is like trying to sand rubber. __________________

Primer helps give a smooth surface because it contains a bunch of filler, usually/often calcium carbonate (note how heavy a can of primer is). That filler flows into imperfections and helps smooth things generally. In days of yore, it also made a primer that would sand easily. Them days are pretty much gone :( with a few exceptions.
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dadiOH
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