Hot Rod Forum banner

1 - 13 of 13 Posts

·
Registered
Joined
·
8 Posts
Discussion Starter #1
I bought fiberglass fenders for my 37 as I didnt have any metal ones. They didn't fit perfectly, as I knew they wouldn't, and I found that I could bend them into shape by putting the knipco heater on them or just leaving them in the sun for a while, and using a strap or piece of wood to stretch or compres them, then let them cool off for a few days to set.
My question is i was wondering if i paint my fenders and running boards black, park it in the sun for some time in say 100* heat (which you could probably fry a egg on em at that temp) will screw with them. I made some braces to set them pretty good, but Im just not sure what that kind of temp does to fiberglass. Thanks in advance.
 

·
Registered
Joined
·
1,189 Posts
Folks have been using fiberglass fenders and bodies and hood tops for close to 40 years or more and I have never heard of anyone's fenders deforming after spending time in the sun, and here in Texas we have serious sun.
Fenders and other glass parts can be deformed more from improper storage, that is laying them down in a position where gravity can do its evil work.
Paint 'em, bolt 'em on and run.
Whose fenders are they?
 

·
Premium Member
Joined
·
4,097 Posts
Home brew is a psychic. :thumbup:

If you paint them black those fenders will absorb more heat when you lay them in the sun. The surface temp of a black part on a 115* day in direct sunlight is about 195*. (yes I checked this with an infrared thermometer one day)

Orthopthalic polyester resin will withstand about 180 before it starts to change. It will soften from that amount of heat and if it is held in the shape it needs to be while it cools it will hold that shape. repeated heat/cool cycles will normalize it further. A part made from ortho resin will also shrink some during this process so you will have to block out any pattern transfer or "print-through" that will occur.
This is why many manufacturers reccomend that you post cure the part in the sun or in an oven prior to finishing it.

I have post cured many parts by putting them in a cabinet with a heater. heat to 160 or so and let cool 2 or 3 times. This will get most of your curing done. I would not recommend painting anything made of FRP black as a finish color unless I knew it was made with vinyl ester or epoxy resin. Even tooling resin, (isopthalic), will only withstand 212* before it starts to change. (wescotts uses iso resin)

I have seen many panels on cars move around in the sun. I have seen a black decklid move 3/4" inch from morning to afternoon, then back again. The outer panel expands while the inner doesn't, hence the bowing effect.

Fenders don't move all that much because they are typically single sided. They will ripple and mottle some on the surface though in the heat.

Whatever you do don't let the part get so hot that the resin crystallizes. It will do some really strange stuff then.

Later, mikey
 

·
Brian Martin,Freelance adviser
Joined
·
16,328 Posts
powerrodsmike said:
If you paint them black those fenders will absorb more heat when you lay them in the sun. The surface temp of a black part on a 115* day in direct sunlight is about 195*. (yes I checked this with an infrared thermometer one day)

Later, mikey
I was told that the COLOR will affect the reading on the infrared thermometer bedamned the temp of the surface.

In other words, "shooting" a white car and a black car sitting next to one another my be deceiving because the actual surface temp my not be the same as the infrared thermometer is reading because you have shot two different color which reflect color at it.

Can you do a little test Mikey? Get a piece of metal and paint it black on one side and white on the other. Put it in the sun for a while with the black side up. Check it's temp on the black side then flip it over and check it's temp on the white side. They should be the same.

Brian
 

·
Premium Member
Joined
·
4,097 Posts
I can redo the test with my fluke electronic thermometer, a thermocouple and an infra red heat lamp. I think we are out of the 100 degree days for a while.

I know when I did that little test I was playing with my neighbors infra red thermometer. I also shot the building inside and out, (it is beige), and the temp was the same. 135* inside and out.

I would think that a test with the undeside painted white would be inconclusive, as the white would reflect the heat back up into the steel. That would be my guess, anyway.

I have an old fiberglass fender still in black gelcoat that I can drill a hole in the center of the laminate and insert a temp probe with some silicone heat transfer grease. then I can subject it to my vast array of mad scientist materials testing equipment. :evil: MOOOHAHA!

See you on creature features,
Mikey
 

·
Brian Martin,Freelance adviser
Joined
·
16,328 Posts
Mikey, the white won't "reflect the heat" back because the color doesn't "reflect heat" it reflects "color". The color white is actually reflecting all colors of the spectum back at you. The color black is asborbing all colors of the spectum, along with all the heat it carries. So white on the back of the metal "should" be exactly the same temp as the other side of the metal.

Brian
 

·
Premium Member
Joined
·
4,097 Posts
Edit- I re read your post and understand now,

Ok, You are correct in that the color won't reflect the heat itself, but in reflecting the light, it reflects the heat that is carried by it.


You are saying that the temp of 195* that I got when shooting the surfaces during my little test was incorrect due to the thermometers inability to differentiate between colors.


But you do agree that a black coated part will absorb more heat than a white one.

I do know from experience that a black part will be more inclined to change from that heat than a light colored part, due to the resin softening. I know the temperatures at which that occurs, so I never questioned the temp readings of that infra red thermometer.

Brian, what do you suspect the actual substrate temperature of a black part in direct sunlight on a 115* day to be?

Later, mikey
 

·
Registered
Joined
·
5,653 Posts
:D

I have NEVER had this problem ... on my 32's ... :D
But most fiberglass companies tell not to paint their bodies BLACK ..
 

·
Registered
Joined
·
8 Posts
Discussion Starter #12
Thanks to all for the good info.
Theyre Pros Pick fenders. The people there said theyve never had a set warp from being black. I was just curious after the countless hours spent heating, bending, cooling, if anyone had heard of sunlight problems before. Pros Pick also set me up with bed wood, strips, & a tailgate. Good helpful people.
 

·
Brian Martin,Freelance adviser
Joined
·
16,328 Posts
powerrodsmike said:
Brian, what do you suspect the actual substrate temperature of a black part in direct sunlight on a 115* day to be?

Later, mikey
Mikey, I only have questions, few answers. :) I have no idea, don't understand much about the science of it, I have heard that there would be a difference.

My brother has an infrared thermometer and I can give it the test to see.

Brian
 
1 - 13 of 13 Posts
Top