best approach is to start by checking the instructions on the can or the interlux web site.

that said, for the interlux's, at least when you get to the final colors, the approach that has worked consistently for me is to clean once with clean water/clean cloth to remove any dust/larger particles and then a final wipe down with paint thinner to get any other contaminants off....then put a thin coat of paint down.   when it is dry....put a thin coat of paint on.

i would avoid any 'soaps' or other chemicals other than clean water and a final solvent wipe as recommended by the paint manufacturer.

biggest issue i typically see is trying to put a second coat on a not-fully dried undercoat or applying too thick a coat in one session....which will get you some interesting orange-peel type problems.  

the other issue i have seen is accidental contaminants from other stuff in the work area (oil for example) or in the cloth you are using to wipe with (e.g., fabric softeners).  To keep it simple,  i usually just use white paper towels.

fwiw, a light wet-sanding between coats and a bit of patience to get all the coats you need can really lead you to a very good looking finish.

Thank you very much. The can directions call for special solvents and washes, but try and find them. Thank you again for pointing me in the right direction.

your right....the cans call for special solvents and washes....but if you look at the chemisty of what they are asking for, it typically comes down to some inexpensive common commodity chemical that is easy to find.

if you are not comfortable sorting out the chemistry....web searches will often lead you to a boat-building site where somebody has already figured it out.

i don't usually buy the vendors specialty 'solvent' to avoid having a bunch of 'private label' cans of noxious chemical around the garage, but use its commodity equivelent.  for the interlux brightsides paints, its paint thinner/mineral spirit wipe between coats.  between coats of epoxy its denatured alcohol. 

i am not personally familiar with the under coat you mentioned....so that is why i suggested you refer to the interlux site.  sometimes under coats are substantially different than the top coat.

my sense is that the manufacturers do try to make it a bit more complicated than it needs to be and look to sell a lot of their 'name brand' solvents.  but in my experience, it has not made a difference in working with the commodity item and leads to a lot less clutter in the workshop.