painting roof

[XLD]

Thought I'd rust treat with Owatrol, then coat with red oxide before 2 coats of topcoat semi gloss. As it's going over old paint I assume undercoat is unnecessary, but am I right?

[Higgs]

3 minutes ago, XLD said:

Thought I'd rust treat with Owatrol, then coat with red oxide before 2 coats of topcoat semi gloss. As it's going over old paint I assume undercoat is unnecessary, but am I right?

 

I wouldn't use Owatrol, but that's a hangover from the days when I used to paint boats for other people, and didn't want there to be any question of what might have caused painting to go wrong..., if it did. On your own boat, you do what you want. But I would put undercoat on to even out the appearance of the topcoat. You might otherwise need more of the more expensive topcoat, to hide various colour patches below; especially with reds and yellows. 

 

 

 

[Tony Brooks]

4 minutes ago, XLD said:

Thought I'd rust treat with Owatrol, then coat with red oxide before 2 coats of topcoat semi gloss. As it's going over old paint I assume undercoat is unnecessary, but am I right?

 

Makes little sense to me. What is the Owltrol supposed to do on old paint?  Is not red oxide a primer? So now you end up with a red roof that may well be patchy color-wise so what color will the top coat be? If it is no red/brownish then you could end up with patchy gloss. Undercoat is used to  provide an even and compatible color so the relatively transparent top coat ends up with an even color all over.

 

Without knowing what the Owltrol is for and assuming the roof is not sanded I would scrub well to degrease and abrade the existing coat well, then a couple of undercoats and top coats. If it is sanded and/or if it has rust showing through I suspect to do a good job it all needs to come off and do a proper derust job.

[Higgs]

You can also tell when someone has skimped on the layers, and looks too thin to give the depth of colour or create a sufficiently protective coating. And always finish in the same direction, with brushing, on every layer. 

 

 

Edited May 7, 2022 by Higgs

[David Mack]

If you have a good existing paint finish all over, then you could sand it down and just add a couple of topcoats. But if it is rusty enough to need rust treatment, then you need a full paint system, including undercoat, at least on the rusty areas. In which case it would make sense to undercoat the lot to get a decent even colour.

[Higgs]

1 minute ago, David Mack said:

If you have a good existing paint finish all over, then you could sand it down and just add a couple of topcoats. 

 

If the paintwork is thick, it might be possible to end up with an even layer to paint on. Evenly coloured, that is. Otherwise, the whole surface will need an undercoat. 

 

 

[Higgs]

Wrong thread. Ahem. The thing that was here, that I've now removed to the correct thread. 

 

 

Edited May 7, 2022 by Higgs

[blackrose]

7 hours ago, Tony Brooks said:

 

Undercoat is used to  provide an even and compatible color so the relatively transparent top coat ends up with an even color all over.

 

 

And the other function of undercoat is a tie layer between primer and topcoats. Paint topcoats direct to primer and they may not stick. 

 

Primer/undercoat/topcoat.