#1 All paint should be removed, every last little bit.
#2 Measure moisture in substrate using a moisture meter. The hull should be as dry as possible before barrier coating. Proper drying can take months this is why it is a good idea to strip the hull in the fall, for Northern sailors, and let it sit over the winter. Coating a wet hull is pointless..
#3 Any blisters should be excavated filled and faired. I prefer to fair with a mix of coloidal silica/milled fiber/epoxy. Microbaloons are easier to sand but can absorb moisture, though under a proper barrier coat this should not be a big deal.
#4 fair the hull and prep for barrier coat. It should be as smooth as possible.
#5 Despite sanding you should still de-wax the hull with a chemical de-waxer like those from Pettit or Interlux.
#6 Choose your barrier coat. I prefer Interprotect 2000, as do many professional yards because it has long "hot coat" windows, is easy to use and comes in two colors so you can alternate and know your not missing areas.
#7 Always try and hot coat the layers of barrier coating. This results in a chemical cross linking and avoids sanding and washing between coats.
#8 You'll want to "hot coat" the paint to the barrier coat if using IP2K. The hot coat test is when you can leave a thumb-print, it is ready for the paint. You'll need two people as "thumb print" happens very fast, as soon as 15 minutes, so one guy will be rolling the last coat of IP2K and one the first pass of bottom paint. Hot coating the bottom paint is perhaps one of the most critical aspects of the job other than properly drying the hull. Do not skimp or miss your window on this or you'll be applying yet another coat of IP2K so you can hit the hot coat window.
If you need to sand and wash the hull you will not get the adhesion you get with hot coating. If you miss the thumb-print window simply apply another coat of IP2KE and don't miss it the next time or it will cost you another $100.00.
If you follow the above standard guidelines you will have a bomb proof barrier coat with phenomenal adhesion...
Above all else read the manufacturers instructions for temp and re-coat times..
This hull could have been slapped with barrier coat but the yard is doing this correctly and drying the hull. The drawings on the hull are moisture meter tracings with the dates and meter readings..
Many unscrupulous yards will gladly take your money and overcoat a wet hull. Any yard that says they can haul, strip, fair & barrier coat, South of CT where boats stay in the water for years on end, and do it in a "few weeks", without using heaters, is full of BS.
This unscrupulous money grubbing behavior is what has lead to many unwary boaters thinking their barrier coat was not water proof or failed when new blister appeared. The blisters & moisture were already in there but just trapped by a fresh barrier coat by a corner cutting yard. The moisture came from inside not outside..
Originally Posted by Interlux
Note:Overcoating times will vary due to wide variations in temperature and humidity. The best method is to check the paint film using the "Thumb Print" test to determine when the Interprotect 2000E is ready to overcoat. If the primer feels tacky and you can leave a thumb print in the paint film without getting any paint on your thumb the Interprotect 2000E is ready for overcoating.
Continue testing every 15 minutes using the "Thumb Print" test until reaching the Ready-to-Overcoat stage, then begin to apply Interlux antifouling immediately.
Do not apply if the relative humdity exceeds 85% or if the temperature is above 95°F (35°C). Check primed surfaces using the "Thumb Print" test before overcoating with antifouling.
If you cannot leave a thumb print in the Interprotect 2000E or it is "tack free" it is too late to overcoat and must be removed. Follow application instructions on the antifouling label for overcoating times of subsequent coats.
Originally Posted by Interlux
Some Important Points: The hull must be dry. Do not apply over one part paints. Product temperature should be minimum 10°C/50°F and maximum 29°C/85°F. Ambient temperature should be minimum 10°C/50°F and maximum 35°C/95°F. Substrate temperature should be minimum 10°C/50°F and maximum 35°C/95°F.