Blocking off part of mold

I want to create a walkthru and block off part of mold with mdf. Also,creating a rebate for deck to hull exterior reinforcing overlay.
What sealant would be appropriate to stop the resin running underneath the mdf. Mold will be waxed with TR108 x 6 coats.
MS,Water based acrylic,or ordinary silicon?I have read to keep silicones away from the mold. Can't recall why.
I have noticed in the past mdf inhibits the cure of polyester. Maybe 6mm ply would be better for the tabbing rebate?
Thanks for any tips, tricks.

 

Roly, I use melamine faced particle board(like in kitchens) for that kind of thing, & use small dabs of hot melt glue to hold in peices, cleats, brackets to screw off to, & use plasticine from the glass/resin supplyer to seal/ roundover the joins of internal corners & wax & realease as usual, to external corners I'll leave the melamine short & square out with car bog sand & router to radius, some times I'll use brown packing tape over this, it only leaves a tiny step if appplyed carefull(only to intersecting flat planes). Lamipanel(3mm) and poly board(4.5mm ply with smooth white face) make good tabbing rebates to mold surface, years ago we used to use masonite+shellac & johnsons wax for temp molds & got some pretty nice results. Also be carefull with the hotmelt glue it varys in quality & some is nearly toooo! xxxxing good- do some test. Regards from Jeff. PS: 5c 10c & 20c peices are a nice AUS/NZ standard radius tool for plasticine- roll into thin sausage, heat gun to soften in position thumb in then tool off with soft heat & coin, razor off train tracks beside radius & wax & realease with pvaBe carefull of car bog too some brands are bad for gelcoat/resin cure- lotsa wax, pva, packing tape usualy make it OK

 

Jeez mate! Bloody awesome! Couldn't have got more detail off a boatbuilding course.:idea: :idea: :idea: love the plastercine & heat/coin, & melamine for a good finish.
Thanks heaps Jeff.! I gonna print that out and glue to the shed wall!

 

No wurries Roly, just hope the "slushies" guild don't come after me for lettin' out all them "secrets" Try to make your mold as neat as' cos it saves more time than it costs in finishing details if you mold it nice to start with- the lazy man works twice as hard in that regard. Have fun thinking inside out! Regards from Jeff