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Building Tips and technical articles. => Paint and finishing => Topic started by: James_Mynes on September 09, 2013, 09:34:52 AM
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This is the first time I've EVER used primer on a model. After reading the post above by Randy Ryan, I felt good about my choice of Duplicolor FP 101 Filler Primer (High Build Formula).
A day after spraying I hit it with some dry 320 grit sandpaper. The paper fills up almost immediately, so I switch to wet sanding, still with 320. Now the paper doesn't fill up, I'm getting some of the primer off as evidenced by the gray sludge and spots of transparency showing up. So then I stop to admire my work. Wipe off the area with a towel and hold it toward the light, and I see a very glossy surface where I've been sanding. This alarms me, as I expected some dullness, some 'tooth' for the color coats to grab onto. Am I doing something wrong?
My plan is to finish with Sig Dope after I get the sanding thing sorted.
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Well, I generally lay on the primer pretty thickly over 2 or 3 (or perhaps 4) coats of unthinned non-taunting clear. I do this because I hate having to go back and re-shoot area that didn't fill a low spot. A quirk, what can I say.
You need to use a non-loading sandpaper. I like 3M "Sandblaster" paper. The stuff will load a bit but a flick of your fingernail will clear it. I use a block and usually some 220 paper to take it down a bit then switch to 320 to finish out and get rid of the scratches from the 220. Then I usually hit the whole airframe with some 800 to make sure most of the scratches are out. All of this is dry sanding. Then I shoot a coat of very heavily thinned clear (like 80% thinned) over everything to seal the primer though there is usually not much primer lest at this stage. Then I use some 0000 steel wool to make sure all the dust is out of the last coat and I have a smooth surface. Not slick, smooth. Then I start shooting color (as I'm just about to do with my new plane).
Be sure to clean everything really well between coats, especially after handling the thing so much while sanding primer. I use Windex as recommended by Phil "Mr. Concours" Granderson.
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Is it a bad idea to ever lightly use a sticky tack cloth? I've seen it recommended in kit instructions, but worried that it leaves some of the stickum behind.
Rusty
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Rusty,
I use a tack cloth every time I sand the last coat of primer. It will not leave any tack behind if you use a light pressure when swiping the cloth. I use gloves to keep the tack off my fingers and try not to touch the airplane unless I have a clean rag in my hand. I immediately wash the surface with Windex and then paint.
Paul
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Jim, I learned sanding on cars, where you used a hose set to "dribble" to put the "wet" into "wet sanding"*.
If you are using proper wet or dry paper, and 320 grit leaves a gloss behind, then you're either not cleaning the paper often enough (you should swish it around in water often enough to keep it black, if you're not using that dribbling hose) or you've allowed your paper to get dull. Paper does get dull -- if you're wet sanding, dip it in water, feel it, then sand with it for a while. When the paper starts feeling significantly smoother than it did when it was fresh, it's dull and you need to start using a fresh patch.
* When my dad built the front building for Wescott's Auto Restyling in the 1950's, he had the concrete pad built with a slight slope down to the parking lot -- something like 1/4" to the foot. So you could wet sand all day and the water would just drain out.