Ok I disassemble the case removing the HDD rack. I cut out the back panel to fit the two 80mm fans.
For painting "metal" i like to use "self etching Primer" as it grabs the metal better. Now most cases are painted or plated with some coating to prevent rusting. The coating is MUCH better than what most of us could apply so I DONT sand it all off.
What I like to do is wash the parts with mild dish soap to remove surface grease (remember every time you touch the metal with your fingers it leaves a bit of grease)
try to not handle the parts too much, I like to barely touch the ends/corners.
After they dry I scuff them a bit with some 320grit (try to get the corners too) you dont need to work crazy hard here just scuff the surface to give the primer something to grip to. Now I put on a couple of coats of primer. There are different colors of primer (gray,red balck)too so if you are gonna paint your case white dont choose a black primer, lol
READ THE CAN DIRECTIONS, this is one of the most important things. If it says paint in 70 degrees F. try to paint in 70F(as close as you can) also do not use cold paint to paint a warm part. If the can feels cold set it in a bucket of warm water for 5 min to warm it up (sprays better) and if it says several light coats are better than use a few light coats (if the paint runs thats a VERY heavy coat)
A Very important Part is to let the paint dry first. I am always impatient and usually dont wait long enough, in the best case scenario will let the top coat scratch easy and worst case the top coat may dissolve the primer and make a mess.
After the primer dries I lightly sand it with about a 400grit (I use what I have laying around, 320,360 400,600) Than its time for the color coat.
There are basically 2 types of paint we use in cans Enamel and Lacquer. Enamel dries slower and is thicker but tends to be more glossy. Lacquer is duller but dries faster (much) and goes on thinner. If you spray lacquer over a Enamel Primer you need to MAKE SURE you let the enamel primer dry or the solvents in the Lacquer will eat up the enamel. Enamel uses a milder thinner and will not ruin lacquer primer as easy. So I like to use Lacquer for the Primer (read the can because a bunch of primers are Enamel) If you want a shiny top coat and you use a Lacquer for the color coat that you will probably need a clear top coat (again make sure the paint is dry before covering it with another paint)
Some people sand the top color coat (if using Lacquer for the color coat) before adding the clear, if you do make sure its dry and us a very fine sand paper (like 1000 or 1500grit) sand it lightly(just enough to get rid of the dust or roughness) This works with Lacquer as it dries in about a half hour or less.
I usually dont cover Enamel with a clear because it can take weeks for the enamel to dry.(it dries from the outside in)
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