clear coat

I recently painted my motorcycle. After applying the clearcoat I noticed particles in the clearcoatfinish. Can I sand the clearcoat smooth and recoat.

Reply to
ronkinz
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you should be able to, not two smooth, leave the surface - with a "tooth"(profile) so the clear coat will adhear.

if you will use a _tack cloth_ (available at paint stores) and wipe the surface just

before the application of the clear coat - it should pick up the "debris" from the

surface - resulting in a smooth finish, void of any particulate matter.

m h o =A0v =83e

Reply to
fiveiron

Sure! You should use wet-sanding sand paper - 400 grit for enamel based and

600 grit for lacquer. sand it smooth. When applying your clear-coat you should be inside with as minimal dust/air movement. temp should be around 60 - 80F
Reply to
Backbone

I used to finish electric guitars and other small wooden items for fun, so I know a few things about making a nice finish in the context of a hobby environment.

Firstly, realize that there will always be some particles, unless the air is perfectly clean, and the solvent you are using flows perfectly before allowing the paint to dry. There is almost no such thing as just spraying a coat, leaving it and calling it done! So if you keep sanding and re-coating to get rid of particles, the coats will keep piling on and you will never be done!

You see, when you have decided that the piece has enough coats, you have to cut and polish it.

This means that

  1. You wet-sand it with some fine paper (at least 800). With this paper, you knock down the particles that may have settled on the finish while it was drying, but go gently enough not to go through the coats. Some finishes tolerate more sanding, in particular the acrylic or celluose lacquers which dry by evaporation of solvent. With these finishes, each coat dissolves into the previous one to form one big coat. On the other hand, cross-linked coatings, like catalyzed lacquers, two-component urethanes or epoxies, form distinct layer that are bonded together. If you sand through a layer, it will be visible. If you're a finishing newbie, stick to a good quality, non-catalyzed, one-component acrylic lacquer.
  2. Once you sand it, you polish it with a rubbing compound. This is like sand paper, except that the particles are finer and are suspended in a paste rather than stuck to paper. Rubbing compound will take out the scratches caused by the sandpaper, leaving a high gloss. The compounds do come in different grits also: two step systems. You can rub using a soft cloth and your hand. Or use a buffing wheel. I'd recommend the hand technique since you're only doing a bike (small surface area). And you can brag to your friends that you have a genuine, traditional, /hand rubbed/, finish.
  3. You might want to wait a while before doing the final cut and polish. As in, wait until at least the next day. A few more days is better. Let the solvents gas out and let the finish harden up a little bit. If you were doing the job for someone else, they'd expect the piece bak, but why cut corners on your own bike?

Now about those particles. Some particles are caused by the spray itself: that is called spray stipple. This happens because the lacquer dries too fast as it hits the surface. If the premature drying is severe, you get a condition known as dry spray. Droplets of the paint actually become solid before they hit the surface, and so there is a kind of fine dust most of which can actually be blown or swept off when everything is dry!

The solution for this is to spray at a colder temperature, or to dilute the lacquer with a solvent that evaporates more slowly. There is a number which measures the rate of evaporation of various substances. This scale assigns the value of 1 to butyl acetate, which, incidentally, is a substance that comes in handy here. Acetone, a common component of lacquer thinner, scores 100, evaporating way faster. Toluene, another lacquer thinner component, evaporates more slowly somewhere in between butyl acetate and acetone. To slow down drying, dissolve the lacquer with more butyl acetate and less lacquer thinner. You can buy the stuff separately in many paint places where lacquer thinners are sold. The hotter the ambient temperature, the more butyl acetate you mix in. Mix in just enough so that when you spray the article, you get a nice, smooth, wet coat without stipple.

If you don't eliminate spray stipple with a slower drying solvent, you will just have more sanding to do. It can come out that way.

The second source of particles is crap in the air which lands on the finish as it is drying. And that includes insects! This isn't a terribly huge problem for fast-drying finishes. It can be quite bad for the slower curing coatings, but you would use those only for base coats that get sanded, anyway. E.g. on bare metal, I'd put a coat of epoxy. That would take time to harden, with bugs and dust landing on it. Who cares; it gets sanded. If you are dealing with a fast-drying finish coat, what you can do is wait a few minutes for it to become tack free, and then using a tack-rag, remove any crap that has landed on it. A tack-rag is a cloth that is saturated with a sticky substance, such as a partially cured varnish. You very, very lightly, with no pressure at all, touch the surface with the tack cloth, the idea being that the crap which landed on your piece will stick to the tack cloth and be pulled off the piece. This isn't all /that/ useful with fast-drying finishes, but worth a try. It's more useful with slower-drying ones which still have a chance to level out once an embedded particle is removed, while at the same time they are less prone to new crap landing on them. E.g. varnishes that dry in 1 to 4 hours. (Garbage you wouldn't use on a bike! Or even on any nice wooden item).

Another source of surface defects may be crap in the air line to your gun! Compressors that have oil-lubricated pumps can put oil into the air lines, which is why shops use filters on air lines. Oh yeah, and water condenses in the compressor's tank due to the pressure changes. That can make its way to your gun.

A clogged gun can spit. Guns which use gravity feed can spit too, because, depending on their design, it's possible for the paint to flow to the nozzle even when there is not enough air flowing. Leave that sort of equipment to the pro shops and use a traditional gun with a can on the bottom and a straw in it. In this type of gun, the flow of the air past the nozzle is what what draws the lacquer out of the gun, so it's not possible for a droplet of paint to form on the nozzle when there is no air flowing, and then later be blown onto the finish. When you are done with a finish job, always disassemble the spray gun and soak the parts in solvent so none of its passages are clogged. Before you spray, take the can off the gun, and dip the straw into a jar of solvent. Squeeze the trigger and let some of that solvent spray through. Then remove the straw from the jar and keep spraying until it's empty.

Inadequate pressure can cause poor atomization. You have to use a high enough PSI. If your compressor can't crank ouf the CFM (cubic feet per minute) needed by the gun, typically around 11, you can only spray small areas at a time and let the compressor recharge. (No problem for a motorcycle, with its small surfaces). If you keep continuously spraying with a weak compressor, the tank will drain and the pressure at the gun will drop, resulting in "barley" going onto your piece.

Inadequately dissolved finish material can also fail to atomize properly.

Test on some throwaway part, then hit the real article.

Lastly, drain the water out of your compressor's tank when you're done!

Good luck.

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Why don't you go to Usenet groups? That's where your posting is going from this "automotive forums" portal.

Reply to
Kaz Kylheku

Are they in the clear itself or are they on the top of the color coat and under the clear. Either way your going to be sanding back to the color coat if you want to do it in one coat and not have to start over again. Just sand using 400 until you remove the clear, then go over it with 600. Tack it off good and then to reduce the problem with dust/crud use a small sprayer with some water and spray down the area your painting in. If your only doing some small parts make a booth out of cheap poly sheeting hung up with some clothesline and clips. Make it big enough that you can move around the parts. Then clean the parts and spray.

Reply to
Steve W.

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