caunyd

Hi guys, I have an all black Trek bike. I just want to paint red highlights/trim on it. Since I am just painting on highlights, rather than repainting the entire bike, do I have to sand down the black areas and add primer where I want it repainted red? Or can I just paint over the existing black paint?

What's the downside of just painting over the old paint?

SlimRider

First, wash and rinse your bike. Nobody should have to say this, but apparently its required to say, for a few folks.

Slim's Paint Method:

(1) Place tape over the margins of the areas you want to paint. This will leave the area you want to paint, exposed.

(2) Now, lightly sand (300 grit) the exposed area, being very careful NOT to rip or disturb the tape.

(3) Now, steel wool the exposed area with 00 steel wool.

(4) Wipe down with 100% denatured alcohol and wait a half hour for it to dry.

(5) Use autobody grade paint primer and with a fine haired paint brush, paint primer on the exposed area.

(6) Wait 24 hours.

(7) Lightly steel wool the exposed and primed area again with 00 steel wool.

(8) Wipe with alcohol again and wait for it to dry. Meanwhile, clean your paint brush with paint thinner.

(9) Now, paint your exposed area that's been primed, with the intended autobody paint color.

(10) Wait 24 hours. Steel wool it with 00 steel wool again and wipe it down with alcohol once more.

(11) Paint it one more time after the alcohol is dry. If you are using hi-glosspaint, you're done!

(12) If you're using flat paint, then wait 24 hours and clearcoat it, after you wipe it down with alcohol and the alcohol has dried.

* Wait at least three days before riding your bike (preferably five days).

Wear gloves whenever handling your bike to avoid skin oil contamination.

Snydermann

If your substrate is sound, intact and undamaged you don't have to use a primer. Primer promotes adhesion of the new finish between dissimilar substrates and fills imperfections. If you have a solid one-color surface that isn't damaged you don't need primer, as new paint will stick to old paint.

Degrease and dewax the surface before doing any prep work. If you have wax, silicone or oil on the surface sanding over it can grind it in and cause fish eyes. Before painting you will need to break the gloss off of the original paint. I just use a Scotch-Brite pad until the surface is evenly dull, the pad won't risk scratches like sandpaper may. Make sure you mask any areas you aren't going to paint before sanding so you don't damage the finish you aren't repainting.

tl;dr

Clean, mask, degloss, paint

SlimRider

Couple questions:

(1) If you sand and steel wool without degreasing or dewaxing, won't you be accomplishing the same affect by wiping down with alcohol, before painting?

(2) When using different paints, should you assume that the last paint used is of a similar chemical base. Wouldn't it be better to prime the area anyway, first?

I've always done it this way and never suffered any fish eyes. The finish has always been perfect.

I am certainly not a professional, I've just done it alot, quite successfully as a hobby...