Rotten cills - Fibre Glass Resin

I have a property with some expensive Everest Aluminium windows, sadly some of the cills area a bit rotten, after filling with ronseal 2 part filler, and painting - after 5 months of the south facing summer sun it has cracked the paint, and the filler has lifted. It gets very hot in the summer at the front of the house.

This Thread explored the painting aspect:
https://www.diynot.com/diy/threads/exterior-brown-paint-water-based-or-not.538778/

We have GRP Roof resin, fibre glass and top coat, I did ask the question as to whether this could be applied:
With regards to use GRP for the sills, I would be worried about the sheets delaminating and letting the water completely ruin the timber sills. I have seen people use lead sheeting in the past. In each case the lead has simply masked the deterioration.

As long as the seal between the frame above and the resin is good, this would act as a 'cover', and be better at dealing with the suns rays.
I see Cure-It do brown coloured top coats.
Just wondered if anyione had any other thoughts.

Other than cutting out and inserting timber, you are more of less stuck with filling or covering the sills, did you use the wood hardener first?
Given what a pain the sun can be on paint (my old house front faced south), I would look at overboarding in UPVC that will stay white pretty well in sun.

Other than cutting out and inserting timber, you are more of less stuck with filling or covering the sills
The GRP roofing products are designed to be in the sun, so would have thought that a good solution ... is that the covernig you refer to ?

did you use the wood hardener first?
No, only just come across this product.

If it has fallen out worth treating the timber with hardener a resin that soaks in.
GRP I have not tried, UPVC is easy to work with and inexpensive.

The wood hardener is good stuff, stiffens up rotten wood well.

The mitres on the external cills have opened up, taking the filler with them, water has been penetrating through - normally a lead flashing would stop water penetration into the bay barrel brick work below - however the windows were fitted without any flashing, and suspect the Everest installers at that time (yes thats the one: 'Everest you only fit the best', or is now 'Everest 2020 Ltd') had removed the lead that would have been under the previous cills, maybe to get £25-£50 privately as a side line ??? ... so water has penetrated the brick work, soaked the render - which has now turned to mush and is peeling off with the paint, and stained the internal walls.

A very poor job, a load of damage for such a thoughtless act.

We are going to resin roof the cills and use an appropriate edge sealant

Thanks for the update

Dry flex from repair care. Resin and new wood needs fitting. Watch youtube


https://www.repair-care.co.uk/product/dry-flex-4-3/