You dont prime under stain, Cedar shingles should not be painted. Are
you sure its paint. Hot surfaces are the worst to paint on. that may be
your failure or freezing. Olympic is getting the highest ratings , read
Consumer reports. If you can remove all loose product stain it . Old
unpreped oxidised wood can peel. Are shingles rotting at bottom lip.
They only last so long. Its a PIA . I have cedar shingles also.