Caulk & Matt Emulsion - How to Prevent Cracking?

[bodgerd]

Help - I tend to use Polycell Flexible Gap Polyfilla as a caulk, and usually let it dry before over-painting. I have had problems with Matt emulsion cracking - any advice? I read on another thread that painting the caulk with an oil-based undercoat might be the solution, but can I paint matt emulsion over this OK? I was planning to use this method on a vertical corner of 2 rendered and set walls painted in Matt emulsion, where my extenstion meets the original outside wall of my house. Do you think this will work, or should I use a different type of filler in this situation? Many thanks from a real novice!

 

[misterhelpful]

Another post earlier today had a similar situation with matt cracking on their caulk. They used oil based undercoat without success which is unusual because it accepts virtually any topcoat, including matt emulsion.

Caulk should be ok, but if the cracks are large and likely to move (which I would expect in this case), you could use a flexible filler like this: http://www.trade1st.co.uk/productdetail/Toupret/103.aspx before using a highly flexible sealant like Everbuild 195, which is a paintable silicon acrylic with 12.5% flexibility and should cope with any future movement:
http://www.tbsmerchants.co.uk/cgi-b..._195_Siliconised_Acrylic_Sealant_C3_Whit.html