Painting over burnt sand mastic?

[inwood]

I have recently (about two weeks ago) applied some burnt sand mastic to three windows/doors. I was slightly sceptical when I applied it but it's hardened nicely and clearly does what it says on the tin (or plastic bucket in this case). The frames and doors have been painted so the mastic around the edges of these will be left 'naked'.

However on the two doors there was a gap between the top of the frame and the lintel above which I've duly filled with newspaper (broadsheet of course!) and applied the mastic.

Believing it was sufficiently hard I've now tried painting the mastic (oil based paint) only to find the paint pretty much disappear into the mastic.

Do I need to wait longer for the mastic to dry out before painting or is there a rare magic primer (goose spit/elm sap/frog spawn :wink: ) that needs to be used?

 

[inwood]

No one with any advice??

 

[Geoff Flegg]

No words of wisdom from me, but I'd also be interested if anyone has an answer, as I'll have the same problem very shortly. I was just going to leave the mastic until it had hardended then overpaint with linseed paint.

 

[Ian Bond]

Once the mastic is firm enough to be painted without the brush dragging it, just carry on painting as normal and it should be OK after a primer and 2 or 3 coats.

Previously I've overpainted with F&B Exterior Eggshell. One coat of ordinary oil primer followed by 3 coats of oil eggshell before it looked OK, but I did not wait for more than a day or so between coats.

However, I've now abandoned oil alkyd paints for the exterior, because they're not lasting - peeling off in under a year from the putty, although OK on the wood and mastic. I am extremely disappointed, having put so much effort into preparation and painting.

So now I'm using linseed oil paint. I put the first coat onto the new mastic after about 1 week, and it soaked in. I've not bothered to put any more paint on yet but expect it to take a couple more coats to cover well.

I'm hoping that the linseed paint will stick better, particularly to the linseed mastic and putty. I've not stripped back the windows to bare wood and it appears to be alright.

 

[ian]

I need some reassurance as well. Having now digested burnt sand mastic (not literally but that kebab tasted strange) I want to try it when I get home, if I am allowed near the Aga with a bucket of sand. We have used linseed paint for the new windows and have got quite patient waiting for the colour/ coverage to come out. I suspect it will all come out in the wash as the oil dries and the pigment increments.

 

[NT]

I'd just paint it again with oil based paint


NT

 

[biffvernon]

The mastic will get harder for months. I would paint it with linsed oil paint - next spring.

 

[Ian Bond]

That's exactly what I'm going to do Biff - wait for some nice painting weather next spring. We hope!!

 

[Ian Bond]

Just found this
http://www.bradford.gov.uk/NR/rdonlyres/290A6A75-8EBC-48C5-8D6E-1401C1426EF7/0/WindowsandDoors.pdf

The bit that caught my eye was:


DO NOT
Paint over mastic. This will cause the mastic
to lose its flexing properties more quickly,
accelerating its deterioration.

I'd would hope that linseed paints would bind well to, and flex with the mastic, and help prevent cracks, so perhaps the above relates to modern gloss?

Can't say I noticed any difference in cracking between the mastic whether I painted (with modern paint) it or not. It developed a crack along the entire join with the timber.

Pehaps the new bit of mastic I've done will work better, done as follows?
* Burnt sand, linseed and rosin mastic mixed myself, rather than ready made stuff out of a tub.

* Brush boiled linseed oil onto the masonry / edge of time frame before trowelling on the mastic

* Used linseed paint (on the frame and mastic) instead of modern oil-alkyd eggshell paint.

 

[inwood]

The document doesn't specifically mention burnt sand mastic but I guess the principles are the same.

Despite that I have little choice since the gap is between the door frame and the lintel above.

I'm pleased to report though that after three coats of primer (modern gloss - although good quality one) the paint now appears to have taken nicely. Just a couple of top coats to apply now.

It was suggested to me that an alternative to burnt sand mastic could be bog standard linseed oil putty (for tha gap filling in this case as opposed to mastic around the frame/lintel themselves). Good idea? as I have another couple of minor gaps between lintels and windows/doors to fill.

 

[biffvernon]

It was suggested to me that an alternative to burnt sand mastic could be bog standard linseed oil putty

Anyone tried mixing linseed oil putty with fine sand, say about 2 or 3 parts sand to 1 part putty?

Putty is essentially fine chalk and oil. If this was used to fill the voids between sand grains you might get a stiff, strong but slightly flexible mastic with good adhesion. You might need to add a little extra oil to make it mixable.

I'll try tomorrow but the results may have to wait a few months.

 

[Juju]

Can I use burnt sand mastic to fill and waterproof the gap between our rotting front door frame and the pavement?

A carpenter said there was no point repairing the door frame as it would just rot again and had the idea of either a small step or a small slope of cement away from the door frame & threshold (I've posted before about our problems - at the mo we're flat on the street so any rain pools and comes in under the rotten door frame).

 

[Ian Bond]

Sounds perfect to me. We have stone steps up to our doors, and timber threshold strips to stop the rain coming in.

They don't stop the rain coming in. This is Yorkshire!

However, linseed mastic is used to seal the junction between frame, threshold and stone. I've just helped my neighbour to repair his doorframe using linseed mastic. Mixed to order by me, and not out of a tub from the DIY shed.