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Fine grit in finish12/13/14
I'm spraying a polyurethane paint Milesi in my garage shop "no spray booth".
I understand part of my problem is that I don't have a spray booth, but I'm looking to see if there are any alternative solution...
I'm finding that even though I blow the piece off really well and use a tack cloth afterward, when the next coat of paint dries, I'm still feeling a fine grit in the finish. You can't see it, rather you can feel it. I was sanding up to 320 grit with my RAS from Festool 6 but I'm wondering if I switch to say a Festool half sheet sander, if that will help or is the problem just airborne dust that I'll never get away from without a booth? I didn't work in the shop since yesterday so the airborne dust had plenty of time to settle down.
I also notice that from time to time, a really fine piece of what looks like lint ends up in the finish. Not often at all, but right now I'm spraying a bench seat top and if you look at the right angle, you can see the little spec of lint or dust.
Is there anything else I can do, aside moving to a half sheet sander and go up to 400 grit or maybe even 600 to eliminate the ever so slight gritty feel. It doesn't appear to happen on parts I have stapled "feet" to and the shelves are sitting vertically, instead of flat like a countertop.
Any suggestions would be appreciated..... aside getting a spray booth, because in my current home, that's not an option and the boss at home would kill me. :)
If you are spraying poly I bet your problem is hours of open time allowing contaminants to add up. I would switch to a precat laq or CV which dries to the touch in about a minute or less depending on humidity and temp. I don't have a spray booth either and spray outside with no problems yet of that type.
I'm sure Festool would love for you to buy another sander to solve your dirty air issue in a garage shop.
Contact a Tec from the mfg. of the product that you are applying and make sure that it can be rubbed out. If so you will need to let it cure at least 72 hours then rub it out. Your problem sounds like what Joel said. “Dirty air”.
The rare piece of dust in the finish is difficult to avoid. Clean your spray area very thoroughly with a vacuum, let the dust settle, and then use a garden sprayer to lightly wet the floor. Wear a lint free spray suit and full face mask air supply when spraying. These steps will help reduce airborne dust. When you do get dust, Milesi is a 2K-PU and rubs out very well when you need a flawless finish.
The rough feel of the dried paint is a different concern. That may caused by solvent pop or dry spray. Get a 10x triplet loupe (Belomo or Bausch & Lombe) and find out why the surface is rough. Possible solutions, depending on the cause, include changing your spray technique, adding a solvent that evaporates slowly (e.g., MAK), or applying a clear coat over the paint.
While for years I have had a spray room, I have found a on a few occasions I needed to rig a temporary booth at other locations for spraying items. Could you possibly make a small booth for this purpose? Once we hung plastic sheeting to form walls, used lots of duct and masking tape to secure it. I also used a box fan to force air into my work area as I did not want to draw the spray fumes into the fan for risk of a fire or explosion. While less than perfect, I was able to draw the overspray off my work, getting a much smoother finish in the process. You can also put furnace filters over the box fan to get cleaner air, at least it removes the bugs and big particles. I refinished and clear coated an entire bassboat with really good results for a friend this way. It will also protect your garage from all the overspray.
See attached pic. I will be replacing all of the filters for it and taking someone elses advice to vacuum the parts after i sand them. Im currently using a 6" RAS from Festool but it doesn't take away all dust. I believe this last time, i wiped the piece down with a "clean up" thinner and still ended up with a gritty feel. Could any of it have to do with the sander im using and that a half sheet sander would be better? I'm working on trying to buy one off someone on ebay
With this new info from your last post, "i wiped the piece down with a "clean up" thinner and still ended up with a gritty feel", I suspect you swelled up some wood fiber with that thinner. Some cheap thinners, especially cheap lacquer thinner can have water in them. Give it a quick sand with 220, by hand, after that wipe down and see if it gets better. I use compressed air after sanding. It's faster than vacuuming and I feel it does a better job of getting the fines out of the wood pores on course grained woods.
i would stay away from the tack cloth .i just use air to blow off the pieces.
2K polyurethanes are known to attract dust and dirt. If you keep a high %rh (65-70) it will help keep the static out of the sub straight.
are you wiping with your bare clean hand as you are blowing off? I bet not. try it and you will feel all the grit that is still there. blow and wife with hand till there is no grit left then spray. also turn down you air pressure on the blow. no more than 50 psi.
"are you wiping with your bare clean hand as you are blowing off? I bet not. try it and you will feel all the grit that is still there"
..so true, my procedure is to use a dust brush with a small amount of air when on the spray platform and then give it that final feel with a bare hand, amazing how much crud is still there to be got rid of. Without doing that it's easy to assume your finish has attracted it all when wet.
Don't forget that vacuum cleaners raise at least as much dust as they pick-up. A nice set-up would be to pipe the vacuum's exhaust strait outside.
Some poly-urethanes that have been opened and the can partially used will develop fine, sand like particles from the oxygen starting to cure the finish, even though you close the lid tightly after it has been opened. Be sure to use fresh finish for the final coat. You may have "first coat" material if it has been opened before.