Technical Question for the Pros - Sanding for Paint Prep - High Spots

A question I’ve been meaning to ask for a long time is why is it every time I’m sanding down high spots is that the first thing to disappear is the paint around the high spots?
I’m no professional. Never used a proper paint gun, but have done plenty of rattle can stuff, and 100% of the time I’m always dealing with this issue. Is it my giant fingers, my technique, or is it just the way it is?

Case in point. Was removing some excess epoxy from my exhaust collector, and I’ve reached bare metal (one top coat), before knocking down the epoxy high spots.

 

Mostly just thats the way it goes. You either need to knock those high spots down before you continue (the better way) or build everything up around it.

Hopefully you are sanding with a block of somesort and not just you hand. Your hand is not an accurate backing for sandpaper for trying to make things flat.

 

Using my thumb/hand. Come to think of it, I rarely use a block or anything like that. Did a patch panel repair on one of my OT vehicle front fenders a few years back. Did use a block on sections of that.

 

Your hand is a terrible sanding block and should be avoided. It doesn't provide even or flat pressure to the surface.

 

Are you sure it's high spots if you're riding over the top even by hand it can't be a high spot you need to do more hammer and dolly work.

 

Fingers tight together and held at 90 degrees to the direction of sanding is how I was taught for areas where you can’t use a block or long board. If you can feel it with your fingers you will see it in the paint. Your post does look a little helter skelter, long smooth strokes would be where it is at. I learned this at an after school job in a discount gas/body shop, so factor that into how much weight you give this advice.

 

High spots for sure. Gobs of hi-temp epoxy stuck to the surface of 4” round exhaust collector tube, but it doesn’t matter. Gage the sane issue with body filler. Used some body filler many a time to fill in a low spot, and end up sanding area around high spots first. I need to watch some auto body vids on this type of stuff.

 

it looks like that paper is too coarse

 

Over sanding is the biggest hurdle for beginners
Under applying filler is another.

some high spots can be due to the nature of the dent. Like tossing a rock in mud, you get a ridge around the low area.
To cure that, tap or pull the low up, tap the high around the edge down.

your “high” areas look to be the result of both under applying filler and over sanding.
As the metal around the dent begins to appear near the dent, that indicates that that area is either a high spot or you are done sanding in that area.
if you reapply filler to those spots, sand with the contour of the part. If the surrounding metal starts to show up and the filler indicates it’s still low, stop sanding. Scuff up the low spot and apply more filler.
If when using your hand to check for level and the surrounding exposed metal feels high, tap it down than apply additional filler.
Apply enough to the area that allows the filler to feather into the metal. In other words, wipe an area slightly larger than the dent.

 

Might help to apply filler to the entire area, well past the edge of any dings or waviness. Then sand with a block (maybe 60 grit?), until the filler starts to get thin enough you can see through it in a few places, and STOP sanding it. If there are obvious low spots that the sandpaper didn't contact , apply filler again over the ENTIRE area, and repeat the coarse block sanding.

When you're happy with the filler, then prime with high fill primer, and again sand with a block (maybe 150 grit?) until it starts to get thin in a few places, and STOP sanding. Then prime again with high fill primer, and sand (with 240 or 320?) until the primer gets thin in a few spots.

This is kind of how I do it, and it usually only takes a few tries to get it almost good enough to paint.

Have fun...

 

Epoxy filler is probably a lot harder than rattle can primer and would disappear a lot faster than the filler when hand sanded.

 

As @daylatedollarshort mentioned it looks like you are just sanding at random all over the place. You will never get anything flat that way. If it was me I would use a long flat sanding block and sand up and around the tube. The blue line being your sanding block and the yellow the direction of movement around the tube from bottom of picture to top. Then repeat from the opposite side in the opposite direction or from the same side but top to bottom..

 

In my case, I would tend to agree with @anthony myrick In terms of over sanding and not applying enough filler generally speaking.
The example in the pic I chose, was probably not the best to illustrate my question. To be clear, it’s a 4” dia exhaust collector that I’ve been working on. I applied some hi-temp epoxy around my welds that were contoured to fill in some imperfections. The spots in the pic were various small nicks, dents, and scratches where I applied the epoxy. Used 80, 120, and 220 grit.
In this case, on something that is round what would you suggest? Also, had the same issue sanding the epoxy around the welds for the inlet tubes. In those areas, there is no space to use any tool. So, basically small bits of sand paper rolled up, and using it almost like a toothbrush. First pic after first top coat, and another application of the epoxy. You can see all the bare metal surrounding the areas where I applied the epoxy. Second pic, finish sanding the epoxy before first top coat. Used VHT caliper paint.

 

I can't imagine any filler other than maybe lead holding up to the heat of an exhaust system ?

 

Haha! We’ll see. There is about an 18” flex tube between the manifold on the engine and the inlet to the collector. I think the max temp of the epoxy was around 500F. If it falls off, I’ll be disappointed, but overall not a big deal. Just my ADHD kicking in overtime. lol!

 

To get something flat you need to sand it as a whole not just here and there where you add filler. You will chase your tail forever doing that. That can't always be done but the section shown in your original picture you could.

 

What happens in metal is similar to water. The bump pushes the middle down, but the surrounding area rises. Just like @anthony myrick said.


There are hundreds of videos about block sanding to guide you. Basic rule is to go at 45 degrees in each direction with the panel as it will sit when installed. Human eyes catch horizontal and vertical wave easier. This cuts thru those.

 

I have applied JB Weld as filler on engine blocks that I smoothed the casting and filled imperfections prior to paint, it worked. Bondo on headers? Aint gonna smell good, aint gonna look good. Polyester melts like leisure suit bell bottoms on a hot side pipe.

 

I'm not a "bodyman" in any sense of the word.
But I did have some success with one or two small projects back in my young and too dumb to be scared days. I used primer over Bondo and rubbed the high spots down with a 'stinky brick'. The primer allowed me to sort the high spots from the low spots. I didn't know if it was right or wrong to spot feather primer under feathered Bondo touch-ups....... but it seemed like it worked for me. The stinky brick did most of the work, and I just lightly hand-sanded the final primer coat. I felt like I was making incremental improvements as I progressed to final paint.

 

I’ve used regular JB Weld successfully to patch a hairline crack on the exh manifold (Merc inboard) of my boat. This is High Temp JB Weld epoxy. Not sure if it will work, but it’s not bondo. Bondo for sure would melt. Haha!

 

I found that going in a circular motion. Helps with the “feathering”. Also, starting on the “filler” and working outwards seems to help, but for small areas, not possible.
As a teenager and having done some body work back then, the first thing I would look at when going to car shows, were the rear quarters of the cars. In our part of the country, most cars would rust out around the rear wheel arches, and most of the cars at these shows would have “ripples” along the entire length of the rear quarters, and sometimes the doors and front fenders. You would have to take a look down the length of the car to see them, but even back then it showed me how difficult it was even for professional body men/body shops to make a surface perfectly flat.
I don’t know what fractions of an inch are involved in these “imperfections”, but it’s amazing how one’s hand/fingers can pick up on them.

 

Sorry I didn’t realize this was for exhaust. Once you introduce flex pipe who really cares what the rest looks like, that sort of stuff should be hidden. I also have a thing against diamond plate in case that comes up. I don’t generally extend effort to four doors either, but I have lots of biases and other problems. Maybe a trial run to see if the chosen materials stand up is in order before you spend a lot of time making it look pretty.

 

Diamond plate? How did this enter the conversation? If I have no doors, does that make up for the use of flex pipe? Haha!
True - I should test first to see if it works, but my ADHD can’t wait. Been saying for too long I’m going to start my engine any day now. Lol!

 

Flex pipe and diamond plate are in the same plane for me, just pulling your chain a bit. I will be here in the cheap seats watching to see how you make out with this, in case I can learn something- go man go.

 

Using a block will be the answer to your problem. Using different shaped blocks will make you an expert.

 

I agree. Not one to use a block much. Now I recall that a few years back, I did purchase a vintage pneumatic file at Value Village that was super long. Wonder where it is now? lol!
I did use my relatively new 6” orbital sander on the collector. Started off with 40 grit, and then 80 grit. Did a nice job of taking out some of the high spots around my welds. Was worried though, that it might start to leave a series of “flat” spots along the length of the tube, as you could see the change in “colour” of the metal as you passed over it, leaving a 1/2” wide strip. It ended up not being an issue, but I could see excessive sanding could ruin the circular surface.

 

I use thin wood sanding blocks.
Like the 5 gallon stirring sticks from the home supply place. Cut to whatever length needed
And automotive paint sticks.

 

I have a thin flexible Dura block that conforms to radius’s but you won’t leave finger grooves.

 

Slightly askew here but I always hear about how one shouldn't use their hands when sanding, I am in another campground on this one. Our hands are the most remarkable tools ever made, don't discount them. They can sense highs and lows and curvature irregularities. I used blocks of every shape, size and composition blocking my black PU but I hand finished the fenders, body lines, roof corners, cowl, dash, hood, etc. If you don't know what you are doing stick to the blocks and sticks, but don't naysay the manos that separate you from the animal kingdom.

 

Some good advice, so tell me why I see ripples/waves in the paint on quarter panels, etc. , of so many show cars?