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PimpJuice 11-13-2005 08:04 PM

Why some runs never leave
This question has been bothering me lately. i painted a car the other day, had one run and i sanded it, sanded it, sanded it etc, until i finally hit base.
No that was not my plan.Why is it that a run or sag will show up when you have sanded a panel until it looks completely flat?
You can sand it down, no orange peel, flat white panel. By doing this it should be gone right?
Why can you then buff and it show up in the right angle? it blows my mind.
BUT if you sand too much you hit base and repair entire panel.
Can someone explain this mystery to me.
Someitmes they sand out and are completely gone, other times they "LOOK" sanded out and reappear when you buff the clear.
I dont see a way for someone to know for sure.
I have had this happen over the years, and thought it must be me. is it?
Am i the only one who has ran into this
I have blown through clear hit base, sanded the area well, added more base and then cleared, only for the run to show where it had been even after i had sanded it out to the base.
What is going on with my painting


poncho62 11-13-2005 08:12 PM

Heres a thought.................Don't get a run.................. :D

crashtech 11-13-2005 08:45 PM

In my opinion it's because the fine grits used don't actually straighten panels well, and a run is a fairly large imperfection. If you sanded it with 80 grit it would be gone, but then again so would the clear. On the occasional run, I have better luck doing a kind of shaving action with a razor which has had its corners sanded off. It's a tricky technique though, and I don't know if I could explain it. The razor is kept at nearly 90 degrees to the painted surface and scraped over the run, being extra careful not to gouge the surrounding surface. Once most of the run is removed this way, you can color sand and buff normally. This technique can work very well, but I have gouged up a panel or two learning how. A big part of the trick is not to forget to dull the corners of the blade.

beemdubya 11-13-2005 09:06 PM

I like to wrap masking tape on each side of the blade leaving about half an inch of the blade in the center.. this way the edges dont gouge at all.. works good.. then put ur paper on a block and do a little more flattening if need be.

PimpJuice 11-13-2005 09:10 PM

i have used the razor blade before. do you think it is from using 2000 grit paper only.
i thought once sanded, it was sanded. this not the case then?
so, 1000 grit would flatten the run, where 2000 may leave some of the shape behind?
i am confused.

i have had same issue with sags


Heres a thought.................Don't get a run..................
you paint perfectly do you?


beemdubya 11-13-2005 09:14 PM

U cant really sand it out.. 1500 2000 it all basically does the same.. on the side of the run the clear is not so thick and just wears out faster.. u have to get the run and only the run to that level.. then begin sanding.

MARTINSR 11-13-2005 10:27 PM

To follow the last post, try this. Keep sanding all the way until you sand thru the paint, the very last bit of paint will be the paint right under that run you had. The paint next to that will disappear and you'll hit primer BEFORE that paint UNDER the run. That is the PROOF that the surrounding paint cut faster than the run. Remember, the surrounding paint is three coats or so right? Maybe three mils thickness. The run on the other hand is more like six or seven coats, maybe more. More like seven or eight mils. If sanded across the two, you will sand thru the three mils before you sand thru the eight.

I have to tell you, I was taught by the head painter at work to shave off a run with a single edge razor, it is AMAZING. I am not talking "shaving" like you would shave a sticker off glass or something with the blade laying on the glass and getting under it. I am talking "raking" the blade with it at a perfect right angle to the surface! It blew me away, but it works!

I'll have to take a few photos and make up a "tip of the day".


shoddy_f-body 11-13-2005 11:40 PM

Shaving the run with a razor blade is the way to go. Shave it until you get it level with the surrounding area. Then block it.That way your taking the run down level,then sanding the area evenly. Another trick is if you get a run,wait until it is almost dry, and then recoat it. You will find that another coat over the dry run will not 'melt in' and will level it out a little and make it a LOT easier to sand out.It will leave a dry spot around the area but hey your sanding and buffing it now anyway. :)

jcclark 11-14-2005 05:14 AM

Another point about blocking out a run, remember that the run is thicker
paint so it's not cured as hard as the surrounding area.
You'll get better results shaving off the top (like the others have said)
then allowing more time for it to harden up before sanding and polishing.
The longer it cures the better.

baddbob 11-14-2005 06:55 AM

Jim, I agree with you completely, I usually do the razor trick and apply a heat lamp or let it set overnight, this allows the thicker material where the run is to harden up some and makes for easier uniform sandability. I use a hard block. I made various sized blocks/sanding sticks out of plexiglass which work really well. With a very hard block and only one thickness of paper you can actually sand a run out to perfection with as fine as 2000 grit. The dry time is key-trying to sand a rubbery fresh run out is usually tough.

jcclark 11-14-2005 07:45 AM

Badbob, you are so right about a "hard block" working so much better.
I have a 2x4 block I sand flat on my bench then sand the run with.
Like you said, it will flatten it even using 2000 grit.
I like your idea of plexiglass, I have some in my woodshop.
I think I'll make some blocks just for runs. :thumbup:

roger1 11-14-2005 09:12 AM

I have had good luck using paint stirring stick cut to about 3 inches long wlith 1000 grit wrapped around it.

BondoKing 11-15-2005 10:52 AM

I learned a nice trick from a friend of mine.. He suggested using a DA and 320 grit paper...put it on the head of the run and spin it until you sand it down to the other clear level as Brian has pointed out... I like the razor, but you gotta be super careful as Bob suggested too. If you are not careful with a razor blade you will pop the head of the run off all the way down to the base... Dry time and being able to read the paint are very important.

Just thought I would throw my 2 cents in :D

BTW the DA trick is my new favorite... I tried it out on something someone ran the crap out of, and it saved him having to clear it again... Paint was the flattest thing out of my garage this year, and I didn't even paint it :(


Osh 11-15-2005 11:03 AM

Meguiers makes sanding stones in different grits just for this, also run files in different grades.

Bee4Me 11-15-2005 11:52 AM
This was from a BAD day. LOL..
I used the razor blade here before sanding.

The clear HAS to be dry to do this properly too.
I've never had much luck with soft clear and the blade.
Yeah, It will scare you at first but once you get the hang of it,it's easy and it's actually hard to mess the flat clear up scraping it's the GOUGING you have to watch out for.

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