Slight low spots/waves in primer

[Polish Painter]

Block sanding , hood and deck lid off of a 1965 Mustang , I sanded 80,180 grit applied epoxy primer , applied 3 coats slicksand , blocked 80 and 100 grit, buzzed over with 180. .
Guide coated , felt it many times, applied 2k h-ibuild , blocked 150 , buzzed over with da 180 interface pad, applied 2 coats hi-build again . Blocked with 400 wet on firm block.

When I wet with wax and grease remover and sight down the panel looking real hard I see a few slight waves, low spots in a few areas . Driving me nuts , Sometimes I do well and think I have this thing figured out , but not today.

From this point , what would you guys reccommend ? Seems like I am chasing my tail.
Block some more on those areas with 400wet on firm block? 180 again ? Spray more slick sand ? These spots dont jump out at you , ever so slight , when the light hits it just right with the wax and grease remover you can find them.

Anybody ever encounter this ?? I enjoy this work but sometimes drives me nuts.

[Phil V]

400 grit is too fine to get it flat. What kind of block are you using ? (a rigid hand longboard ?) I never liked those white plastic flexible long boards.
Check closer and see if the low spots correspond between the back bracing on the hood and deck lid. You may be using too much downward pressure when using the hand longboard. Try a lighter pressure with 160 - 180 grit on the longboard. I'm not a big fan of polyester primer ( slick sand, feather fil etc) I personally would use regular 2K high build urethane primer to finish the block sanding.

[Polish Painter]

400 grit is too fine to get it flat. What kind of block are you using ? (a rigid hand longboard ?) I never liked those white plastic flexible long boards.
Check closer and see if the low spots correspond between the back bracing on the hood and deck lid. You may be using too much downward pressure when using the hand longboard. Try a lighter pressure with 160 - 180 grit on the longboard. I'm not a big fan of polyester primer ( slick sand, feather fil etc) I personally would use regular 2K high build urethane primer to finish the block sanding.

I am using durablocks 12, 16 inch. Also wooden handle boards 2 3/4 x 17 1/2 . Dont use a lot of pressure and try to use fresh sharp paper. Kind of frustrating thinking you got it right and to really know you have to sand with a finer grit ( 400 or so ) to be able to look at it with wax and grease remover.

I feel like I have been doing this long enough that I shouldnt run into these problems . Just venting , Thanks .

[Len]

If you're using Slick Sand under your filler primer then the waves are probably just in the filler primer and not in the substrate. Are you using a decent spray gun with a wide fan to apply the filler primer? Chances are, if you're not seeing the high spots in the guide coat when you start sanding, you won't see them in the finish.

If this happened in our shop I would apply a couple more coats of filler primer, guide coat, cut it with some coarse paper like 180 to 220 then go over it with 400 to remove the coarse scratches.

[Phil V]

I am using durablocks 12, 16 inch. Also wooden handle boards 2 3/4 x 17 1/2 . Dont use a lot of pressure and try to use fresh sharp paper. Kind of frustrating thinking you got it right and to really know you have to sand with a finer grit ( 400 or so ) to be able to look at it with wax and grease remover.

I feel like I have been doing this long enough that I shouldnt run into these problems . Just venting , Thanks .

If its an consolation it happens to us pros as well. Some jobs just turn out to be inexplicably a pain in the arse.

[tech69]

I imagine it's your blocking. If you can't see it's probably cause your pressure is pulling it down and areas where it's strongly held up from the backside stays the same which gives you uneven results, or it's probably that you didn't work out the high spots before all the priming.
If it's just the flexibility of the hood then use lighter pressure, if it's the frame you gotta deal
with it before you do anything else.

I learned the hard way that if you have a good hand feel that you use it more so on hoods and decklids that most likely have slight highs. For those you pull the frame away from the skin by cutting the foam or by a slide hammer and gently pull away. I usually don't strip a hood til I have it in order and I use the paint as a guidecoat. If it has one area it will usually have them all over. It really sucks but it pays off. I used to repair them by just using a pic but that opens up more cans of worms. When corrected the right way you then tighten up the flex by finding which edge tightens it up and then spooning it up to suck it in a little. This way it's fixed the right way and you don't have to baby it when blocking. In simple terms, if you don't check the hood for highs, and they usually are there for old schools, there's no amount of filler or primer that will make it perfect. You fix the intrusions from under you then can properly tighten it up which makes it a breeze cause by fixing from below means it will usually loosen up on you anyways, but tightening it this way won't distort a thing. You'll actually make it the best it can possibly be.

[Polish Painter]

If you're using Slick Sand under your filler primer then the waves are probably just in the filler primer and not in the substrate. Are you using a decent spray gun with a wide fan to apply the filler primer? Chances are, if you're not seeing the high spots in the guide coat when you start sanding, you won't see them in the finish.

If this happened in our shop I would apply a couple more coats of filler primer, guide coat, cut it with some coarse paper like 180 to 220 then go over it with 400 to remove the coarse scratches.

I am using a Devilbiss Finishline 3 HVLP Primer Gun to apply primer .