Priming

[seanoge]

I'm a bit puzzled by priming. If I Prime say metal surely then I can use any paint on it afterwards as the paint does not touch the metal. And this should apply to any material. Or have I missed something?

 

[gc1967]

Depends where the metal is - say it was outside you wouldnt paint it with emulsion, you would undercoat and gloss or you would paint it with hammerite or coovar metal paint
http://www.georgecoullpaintinganddecorating.co.uk

 

[seanoge]

Why not paint it with an exterior emulsion? What's the difference once it's been primed?

 

[gc1967]

There is nothing called exterior emulsion, masonry paint for outside but thats for walls. you either would have to use undercoat and gloss or a metal paint ie hammerite or coovar

 

[gc1967]

Depends if its smooth or pebble dash. sand and cement for smooth, sand cement stones for pebble dash, make sure wall is completely dry and apply unibond the wall before rendering

 

[sparkwright]

There is nothing called exterior emulsion, masonry paint for outside but thats for walls. you either would have to use undercoat and gloss or a metal paint ie hammerite or coovar

But there are circumstances where metalwork would be required to have a finishing coat in masonry paint - metal down pipes perhaps, metal wall brackets etc etc.

Admittedly down pipes and the like often look better finished in gloss.

 

[gc1967]

Cowboys / toshers paint metal down pipes with masonry paint, professional use undercoat and gloss or metal paint, take my word if you paint metal with masonry paint it will start cracking and coming off within a month

 

[opps]

I'm a bit puzzled by priming. If I Prime say metal surely then I can use any paint on it afterwards as the paint does not touch the metal. And this should apply to any material. Or have I missed something?

I kinda get the point that you are making, however in reality when looking at porous materials, such as MDF, you cannot guarantee that one coat of primer will have fully and uniformly "sealed" the MDF. I wouldn't want to apply a coat of waterbased primer to MDF and then one coat of oil based gloss. The gloss would have an inconsistent sheen.

With regard to metals, a primary reason for painting is to prevent oxidisation however not all primers are designed to prevent moisture ingress, they are designed to provide a better key for the top coats. If those top coats are porous, eg masonry paint on cast iron, in time the metal will oxidise and as gc1967 says expect the paint to start cracking.

If you want downpipes to be the same colour as the painted render/brick work, get the gloss mixed to colour match.