question Painting matt and eggshell

[fayjo56]

I know this is probably a really daft question, but I have often heard it is best to paint matt onto gloss paint to protect from rust. But should the gloss be sanded down first to provide a key, or would this defeat the object of the excercise. If not rubbed down wouldnt the matt paint just flake off? OR would i be better off just painting on primer, undercoat, matt paint then wiping over with WD40. Sorry if I'm going back to basics but dont want to be repainting the MW every show season:red:

[Big ray]

I have been painting all of my vehicles for the last 25 years with green gloss before the military mat, dont flat the gloss, its meant to be a water barrier, most mat paints let water through........... however, always check compatability first. Paint a bit of scrap metal and see how it dries. Its always worked well for me, but I am sure that lots of people on here are much better qualified to advize you.

[Pete Ashby]

.. however, always check compatability first. Paint a bit of scrap metal and see how it dries.

 

Ray makes a good point about checking compatibility first on a piece of scrap.

Iv'e found that with most gloss coats using synthetic thinners they must be fully dry and hard (this can be up to 24 hr depending on ambient temperature) before the top coat of drab is applied.

 

If you get a bit eager then some very strange bleed through effects can occur. If you are using cellulose thinner then the surface hardening time can be shorter but the undesirable effects can be more dramatic including complete pickling of the base coat so as Ray says run some test plates using the same primer as on your vehicle.

 

Having tried all sorts of combinations over the years Iv'e come to the conclusion that a good zinc based under coat followed by gloss not flatted then 2 or 3 top coats of drab is the way to go.

[rambo1969]

I have my paint mixed up in satin finish, never had any probs with it.

[Sean N]

I've never had any issues with matt paint. What makes the paint matt is an agent added to the paint which increases the surface roughness of the paint and changes its refractive index. The paint itself isn't any different, as long as you're comparing like with like. As far as I know there's no reason to believe that a matt paint should inherently be any more permeable to water than the equivalent gloss.

 

I'd have thought that a good paint system, well applied, should be effective at rustproofing irrespective of whether it's matt or gloss. Certainly I've never had issues with matt systems, but I have had frequent issues with paint applied (not by me) over unkeyed gloss finishes.

 

If it were me I would (1) get rid of as much rust and rusty metal as possible by mechanical means, (2) use an acid rust converter to make sure any remaining light surface rust is killed, (3) etch prime, (4) zinc phosphate prime, (5) primer surfacer if needed (6) guide coat then flat back if needed, (7) as many topcoats as needed