I can tell you what I do: Usually I sand my high build to get the finish to where I want it. Then it gets a little more complicated. I do a final sand with 400 grit, then I spray PPG JP331 (white or they have darker colors if needed) sealer on that surface. I usually wait about 30 minutes before I start applying my two or three base coats. You could skip the sealer but it is cheap insurance to prevent any ghosting or bleeding or whatever you want to call it when the color below your base is not all the same and it shows thru. I know people that just use the high build (thinned out) as a seal coat. I have not tried that so I have no idea on that. To answer your question, if you choose not to use a seal coat then the base would have a mechanical bond to the scratches in the high build. Using a sealer, then there is a chemical bond between the base and the sealer. Just a note, if you don't use a sealer you might want to use 600 grit as the sealer will fill in the 400 grit scratches and since your not using it you would be asking your base to do that. But as always, check your data sheet and use the grit they want for their base to bite.