Recently I bought a board of the rack, Warner and when I put one of the side fins in the rear tab wasnt sitting down properly, on closer inspection I noticed white stuff (sanding sludge) at the bottom of the plug. It had gone hard so I used a fin key and scraped the crap out. No probs.
The other problem with FCS is that some fins dont fit flush along the bottom as people are saying. One board I have M5s they fit perfectly then used sunnys and at the front of the fin there would be 1mm gap, no matter how I try to screw them in they never fit flush with the board. It does depend on what fin and board combo used.