The ultimate tool for finding low spots on body panels

[MJM]

During my journey to become a "Master Body Technician" ( humor is a good way to start a question ) I've learned some technic's on how to achieve straight and flat body panels, or so I thought. Those technic's are using your hand to feel highs or lows, blocking, and blocking using a guide coat but, even after using those technic's they don't always yield a true flat panel even after using guide coat and wet checking.

After using my hand, blocking, guide coat and wet checking the panels, they looked good to the naked eye but, they where not truly dead flat. So I found the ultimate tool to check panels for flatness is nothing more that a straight edge.

I placed a 4' long aluminum ruler on my door panel after sanding out the polyester primer. Mill thickness after sanding out the polyester primer is around 100 to 200 microns of polyester primer.

Now to fix the lows..........hhhmmm, more Evercoat Ultra, Dolphin Glazing, polyester primer? Strip the entire door and start over and do better filler work?

Straight edge showing a low spot on the door panel. Low is less then 1/64" low.

 

[MJM]

This thought just crossed my mind, I was planing to spray either two coats of SPI epoxy or SPI Turbo prime over the polyester before sealer.

Maybe that would solve my low spot issues?

 

['68 Coronet R/T]

It depends upon the panel you are working on. Some, like the doors and quarter panels my GTX, have built in curves to them, so a 4' straight edge will show a huge low spot from the front to the rear edge of the door.

Using the longest and firmest sanding block that will fit your panel when removing guide coat is essential. I quit using Durablocks, except for their round and teardrop styles a long time ago because they would get deformed too easily.

Now, I do use a long, flexible aluminum straight edge but I lay it flat on a curved panel and then move it from top to bottom looking for problem areas.

 

[Dean Jenkins]

Splines may be a better tool than aluminum straightedge.
Here is a link (courtesy of @theastronaut ) showing how they are used.
FYI - There may be Black Diamond Splines available in the near future

 

[MJM]

It depends upon the panel you are working on. Some, like the doors and quarter panels my GTX, have built in curves to them, so a 4' straight edge will show a huge low spot from the front to the rear edge of the door.

Using the longest and firmest sanding block that will fit your panel when removing guide coat is essential. I quit using Durablocks, except for their round and teardrop styles a long time ago because they would get deformed too easily.

Now, I do use a long, flexible aluminum straight edge but I lay it flat on a curved panel and then move it from top to bottom looking for problem areas.

Valid points......

I didn't have my Black Diamond blocks when filler work was done so I had to rely on using a 3' Durablock. That was hard to control because the center of the block would lift slightly. I didn't like that so I used a 2' Durablock. I was able to keep it flat to the panel but, as you can see, it didn't flatten out the filler all the way even using guide coat as my indicator. I did go over the entire car using the 16" Black Diamond block which greatly helped leveling out the surface but, that was only a 16" block which from your post above isn't the best for blocking out a large panel.

I've been looking forward to getting the 2" Black Diamond block to finish out the remaining primer steps I have left to do.

I just found it interesting that even though I guide coated and wet checked the panel, I could not see any defects ( lows ) in the panel until I used a straight edge.

 

[MJM]

Splines may be a better tool than aluminum straightedge.
Here is a link (courtesy of @theastronaut ) showing how they are used.
FYI - There may be Black Diamond Splines available in the near future

Yes, yes in deed. I've watched that video many times before theastronaut posted it. Splines are the hot ticket in my opinion for spreading filler on large panels. It just makes sense and I hope you add those to your arsenal of body shaping tools.

 

['68 Coronet R/T]

Valid points......

I didn't have my Black Diamond blocks when filler work was done so I had to rely on using a 3' Durablock. That was hard to control because the center of the block would lift slightly. I didn't like that so I used a 2' Durablock. I was able to keep it flat to the panel but, as you can see, it didn't flatten out the filler all the way even using guide coat as my indicator. I did go over the entire car using the 16" Black Diamond block which greatly helped leveling out the surface but, that was only a 16" block which from your post above isn't the best for blocking out a large panel.

I've been looking forward to getting the 2" Black Diamond block to finish out the remaining primer steps I have left to do.

I just found it interesting that even though I guide coated and wet checked the panel, I could not see any defects ( lows ) in the panel until I used a straight edge.

The natural tendency when seeing guide coat in a low spot is to press harder on the sanding block. If the block flexes it will bend under the pressure and remove the guide coat.
Another thing that causes problems is not changing the sandpaper often enough. As it gets dull (and cheap ones do quickly) you will apply more pressure and can again flex the block or the panel itself.
Best to sand with a consistent pressure and change the paper as soon as it quits cutting.

 

[MJM]

The natural tendency when seeing guide coat in a low spot is to press harder on the sanding block. If the block flexes it will bend under the pressure and remove the guide coat.
Another thing that causes problems is not changing the sandpaper often enough. As it gets dull (and cheap ones do quickly) you will apply more pressure and can again flex the block or the panel itself.
Best to sand with a consistent pressure and change the paper as soon as it quits cutting.

You're on a roll today, two more valid points that I probably unconsciously did when blocking out guide coat. I will pay extra attention during that phase of the process. I hope other new painters are taking heed to this knowledge shared. Good stuff for sure.

 

[chevman]

During my journey to become a "Master Body Technician" ( humor is a good way to start a question ) I've learned some technic's on how to achieve straight and flat body panels, or so I thought. Those technic's are using your hand to feel highs or lows, blocking, and blocking using a guide coat but, even after using those technic's they don't always yield a true flat panel even after using guide coat and wet checking.

After using my hand, blocking, guide coat and wet checking the panels, they looked good to the naked eye but, they where not truly dead flat. So I found the ultimate tool to check panels for flatness is nothing more that a straight edge.

I placed a 4' long aluminum ruler on my door panel after sanding out the polyester primer. Mill thickness after sanding out the polyester primer is around 100 to 200 microns of polyester primer.

Now to fix the lows..........hhhmmm, more Evercoat Ultra, Dolphin Glazing, polyester primer? Strip the entire door and start over and do better filler work?

Straight edge showing a low spot on the door panel. Low is less then 1/64" low.

I use a straight edge a lot, but older cars have very few flat areas, if any at all. The Mustang in this video might look flat, but if you lay a straight edge on any part of the body, only a small spot on the straight edge will actually touch the car at any one time. Older cars have to have a crown for strength, the trick is to make the crown consistant in all angles. Depending on how much crown there is, there should only be about 1/4" or so touching the panel, and the straight edge should just roll over the panel smoothly. There needs to be a crown in all directions. If you pay close attention to the upper part of fender/door/quarter as the camera moves, you might be able to see the crown front to back.

 

[RosharonRooster]

During my journey to become a "Master Body Technician" ( humor is a good way to start a question ) I've learned some technic's on how to achieve straight and flat body panels, or so I thought. Those technic's are using your hand to feel highs or lows, blocking, and blocking using a guide coat but, even after using those technic's they don't always yield a true flat panel even after using guide coat and wet checking.

After using my hand, blocking, guide coat and wet checking the panels, they looked good to the naked eye but, they where not truly dead flat. So I found the ultimate tool to check panels for flatness is nothing more that a straight edge.

I placed a 4' long aluminum ruler on my door panel after sanding out the polyester primer. Mill thickness after sanding out the polyester primer is around 100 to 200 microns of polyester primer.

Now to fix the lows..........hhhmmm, more Evercoat Ultra, Dolphin Glazing, polyester primer? Strip the entire door and start over and do better filler work?

Straight edge showing a low spot on the door panel. Low is less then 1/64" low.
View attachment 28331

That's being super super critical lol....

 

[chevman]

Just trying to show him that it would take a lot of bondo to do what he wants. Straight and level is the goal, not flat was my point.

To get the seams straight and level with the two panels, it requires the edges to be pulled out to help maintain some crown, while shrinking the panels. The front fender rear brace has to be removed to get the fender to match the door the reattach it, and the quarter panel front has to loose to make it match the rear of the door.

 

['68 Coronet R/T]

That's being super super critical lol....

Fred is in another stratosphere when it comes to quality metal work.

 

[theastronaut]

The best tool for checking panel shape is aluminum c-channel- thinner/more flexible for crowned panels and thicker/less flexible for flatter panels. I've been using 1/2 and 3/8" for the F100 to check overall panel shape and flow from the cab across the door, cowl, and hood.


Notice that the c-channel holds a cleanly shaped arc front to rear, and makes it obvious that the cowl is sunken in compared to the door and hood.

After correcting the cowl's shape.

Its also useful when welding long seams to monitor and fix weld shrinkage perfectly. This is a door bottom patch, I had the patch fitting very well with the same front to rear and top to bottom shape as the rest of the door skin for a perfect seam, but when you start welding shrinkage pulls the crown out of it. C-channel shows you how much to planish/stretch each weld dot to get the panel back to the exact shape its supposed to be.

Thinner flat aluminum straight edges have enough flex/spring to hold a consistent arc for checking smaller areas. Notice the difference in crown before/after welding.

After planishing and grinding the weld flat.

Front to rear checks with c-channel. At this point I had planished the first weld dots (marked "1") to restore shape, then welded the second round of dots (marked "2"). This shows the shrinkage/loss of crown perfectly.

After planishing, crown restored.

If you overstretch a weld dot it shows up as a high spot. Simply weld again right beside it to shrink it.

 

[Chris_Hamilton]

@theastronaut Really good stuff there. Awesome info. Thanks for sharing it.

 

[MJM]

The best tool for checking panel shape is aluminum c-channel- thinner/more flexible for crowned panels and thicker/less flexible for flatter panels. I've been using 1/2 and 3/8" for the F100 to check overall panel shape and flow from the cab across the door, cowl, and hood.


Notice that the c-channel holds a cleanly shaped arc front to rear, and makes it obvious that the cowl is sunken in compared to the door and hood.

After correcting the cowl's shape.

Its also useful when welding long seams to monitor and fix weld shrinkage perfectly. This is a door bottom patch, I had the patch fitting very well with the same front to rear and top to bottom shape as the rest of the door skin for a perfect seam, but when you start welding shrinkage pulls the crown out of it. C-channel shows you how much to planish/stretch each weld dot to get the panel back to the exact shape its supposed to be.

Thinner flat aluminum straight edges have enough flex/spring to hold a consistent arc for checking smaller areas. Notice the difference in crown before/after welding.

After planishing and grinding the weld flat.

Front to rear checks with c-channel. At this point I had planished the first weld dots (marked "1") to restore shape, then welded the second round of dots (marked "2"). This shows the shrinkage/loss of crown perfectly.

After planishing, crown restored.

If you overstretch a weld dot it shows up as a high spot. Simply weld again right beside it to shrink it.

Another metal craftsman shows up to the party and I'm paying attention. What thickness is the aluminum c-channel 1/16", theastronaut ?

I'm going to buy a 8' - 10' piece and check my car because.........

 

[MJM]

Just trying to show him that it would take a lot of bondo to do what he wants. Straight and level is the goal, not flat was my point.

To get the seams straight and level with the two panels, it requires the edges to be pulled out to help maintain some crown, while shrinking the panels. The front fender rear brace has to be removed to get the fender to match the door the reattach it, and the quarter panel front has to loose to make it match the rear of the door.

I fully understand what you said, It's was my word terminology that was wrong. Straight and level is the goal, not flat. It's interesting looking at my picture, you'll notice the air gap is straight to the panel and ruler. which says to me the panel is straight, just low in that area.

 

[MJM]

That's being super super critical lol....

It's members like you and many others who set the bar high by putting out quality work. Odds say I will most likely fall short of that high quality goal on this car but, it'll be better due to trying too put fourth the effort to obtain quality work. This is my first project car and I've learned so much that I feel my next two project cars will be even better as far as quality is concerned.

This C-channel technic just posted is great information that further adds in my pursuit of perfection.

You guys did this to me, blame yourselves