Painting over Trowel Mastic

[edi2367]

I have recently started replacing cracked mastic around window frames where timber meets the stone. I have done 1 window so far and used stone coloured trowel Mastic from everbuild. In the spec sheet it says only to use oil based paint to paint once hardened.

Is there any reason why I could not use water based exterior paint?
Is it not going to bond to the hardened surface of the mastic?

Thank you

 

[88v8]

Hello and welcome.

I suppose the manufacturers have their reasons. I'd use two coats of oil primer, then the water base.

Ivor

 

[fernicarry]

It contains linseed oil hence water based paint won't be very keen on sticking. You might manage it when it hardens but so far (1 year) mine isn't much harder then when it came out the tub. Its supposed to stay a bit plastic to take up the movement between the timber and the stone.

Also I don't think its really intended to be overpainted even if the manufacturer says you can. It comes self-coloured in buff for blonde stone and red for pink stone so that very much suggests its meant to be left as is. I think it adds a lovely detail to the join between window and masonry.

That said vast majority does get over-painted. Problem is that decorators tend to take the edge further and further out each time so you end up with a stripe of paint on the ingoes. Best not to encourage them....

If I am lucky to last until the next time external decorations are required here I'm planning to treat the mastic as consumable and rake it out before painting, then remake the bead afterwards. Its quite important in preventing water ingress around the window frame so I think renewing it periodically is a good strategy. I've had to cheat on a couple of windows and put a modern mastic in the gap first to ensure it stays waterproof but there's nothing between my windows and the Clyde so they get pretty well soaked.

 

[AxPop]

Replacing the mastic is my last outdoor diy job of the year as it’s falling out in most places. I’ve got 13nr 6’x4’ windows to do. Can anyone advise roughly how many I’m likely to complete per day?

 

[88v8]

Replacing the mastic is my last outdoor diy job of the year as it’s falling out in most places. I’ve got 13nr 6’x4’ windows to do. Can anyone advise roughly how many I’m likely to complete per day?

So a 20' run on each window, rake out and vacuum the gap, assume/hope you find no other issues, redo the bead in some way that's better than the previous efforts.
One or two a day?
You'll get faster of course.