Painting questions

[bononos]

I'm repainting some metal grills/gate around the house with oil based gloss paint (alkyd something in the description) which was previously painted with the same. All the surfaces are sound and smooth with just some small rust spots seeping through. I assume I don't need primer/undercoat and can just paint right on.

1. What grit of sandpaper should I use to sand the surfaces? Am I sanding it to make clean/smooth or to make rougher for the oil paint to 'stick'? Do I even need to sand it?

2. Can I use water based paint meant for wood/metal which is getting common nowadays without primer/sanding the metal surfaces? Same sanding questions as above - grit and reason for sanding.

3. Also repainting doors which was previously painted with oil based paint. Now I want to switch to water based paint for wood/metal - same sanding questions.

4. How long can I wait before painting another layer without having to sandpaper? Does it differ for oil vs water based paint?

 

[Greenman]

1. Don't really need to sand but the surface shouldn't have any shine.
2. Don't know this one.
3. Changing from oil to latex requires sanding and primer. 150 grit will do the job. If you omit the primer it will fail. I've seen this one done incorrectly dozens of times, and replaced dozens of doors that were ruined because of it.
4. Answer will be printed on the can of paint you use.

 

[pbroussard]

1. Sanding oil based paint is a good idea because it gets really hard, and you're abrading it for adhesion. I always used 80-120 grit sanding sponges. Also it's imperative the surface be clean, no hand gunk.
2. prime previously painted oil based coating to switch to latex paint, even the newer waterborne alkyds with an oil underbody or Zinnser's Coverstain.
3. Consider sticking with oil based, abrade old surface and 1 coat of new oil. It's much tougher than latex, great for trim especially cabinets. If you must have latex, prime with an oil underbody or Zinnser's Coverstain
4. As Greenman said, read the can, generally overnight for oil base, couple hours for latex. No need to sand in between coats unless you picked up trash and want it smoother, as oil will bond to first coat since it hasn't cured yet, and like my Dad used to say, nothing sticks to latex like latex.

 

[bononos]

I found out that water based gloss/matte seems to work quite well for doors even without sanding. No issue with paint adhesion and the smell and cleanup was real easy compared to oil paint. I may have to touch up more often because they aren't as durable but at least the color won't prematurely yellow like oil paint.

But they don't flow well compared to oil paint for thin grilles and the durability isn't as good. Water based paint nicks too easily compared to alkyd based enamels so I stuck with normal oil paint for grilles.