Cover sills

T

TwinPlenum3500S

New Member
I have put on new cover sills and where they sit against body underneath, I seam sealed them, to prevent water entering. I have also seam sealed them to the rear where they meet rear wheel arch. Efectively they are sealed from water ingress. However;

1) The cover sils have small holes drille in them, is this normal and ought they have bungs in? and more importantly;
2) At front of cover sill, there is a huge gap where water can enter from front arches as water is thrown up and simply sit there. The oringinals had rotted due to this problem. I am going to block this casm with Dum Dum, and then underseal to prevent water from entering sill.

Essentially the underneath of the car is water tight as it has been undersealed, and the cover sills were loaded with underseal prior to painting and fitting. However, I hate the thought of water being splashed up and getig into gaps in cover sills and just sitting. Am I right in what I am doing?
 
As a general comment, there are two competing schools of thought on this sort of issue. Many, like you, want to seal up against water ingress in the hope that they can avoid having anything corrosive lying in the space. Others open up drain holes etc in the hope of letting water drain out and allowing air to circulate and dry everything out. Personally i think both strategies are doomed to failure - anything made of steel has an unstoppable desire to return to ferrous oxide!

The problem with the sealing everything up route is that in so doing you also trap the air in the space. As the car heats and cools and humidity changes in the atmosphere you will get condensation occour in the sealed space. This can be surprisingly significant. With nowhere to escape corrosion is then under way. Leaving lots of drain holes clearly implies a certainty of at least some water getting in and lying.

The only solution I have come across which seems to me to have a chance, but not certainty, of success is to fill the space with closed cell foam. This is the type of expanding foam that comes in a big aerosol and is used by naff builders to substitute for skill and block up the holes they have left. The theory is that if the space is full of foam then water can't get in and there is no airspace for condensation to form from. There are a few snags though. Put too much in and as the foam expands it is capable of deforming and bursting the chamber - you need lots of suitable escape holes, a bit like casting risers, for while it is expanding. If you don't put enough in then you will form little air pockets which stand no chance of draining and will certainly rust out. Also when the body flexes (eg going over bumps, hard cornering) there is some risk of the deformation of the chamber crushing the foam and leaving small spaces next to the chamber walls.

What would I do? I think on balance I would stop up any holes that clearly invited road spray entering (like the gap at the front you mention), leave plenty of ventillation holes, waxoil everything and then fill with closed cell foam. I'd adopt the same approach for box sections like the sills, screen pillars, front chassis legs etc as well.

As an aside, Ford used to use closed cell foam to improve the rigidity of their bodyshells!

Chris
 
I am afraid that you cannot stop water getting between the metal surface and the foam. And then, the story will continue as we all know.

I think that the best approach is to have the cover sills screwed on as they were meant to be, and simply remove them once a year to hose out any accumulated mud. It's not that difficult job after all and it will keep them happy and solid for year and years.

All those years i get into the trouble to hose out underneath the wheelarches every now and then. Especially during winter that mud collects in tight spaces and creates rust traps. It worked for me....


Demetris
 
You could always remove the sills, and paint everthing with POR 15, which I have discovered is based on MCPU (Moisture Cured PolyUrethane) coincidently the same technology as the aerosol foam mentioned by Chris.

Then seal up any obvious areas where water gets in, in large volumes, but leave the drain holes in the lower sections so that it can get out.

You then have a void where most of the water which used to get in is stopped, any that does find its way in can escape, and a surface inside which is rust proofed anyway.

This is what I intend doing. 8)



I would never use PU foam on my car because-

1/. If there is any moisture when you first apply it, this will cure it faster. If there is a dry area it will cure slower and can seal a pocket of uncured foam within itself i.e you will have cured foam on the inside sill, cured foam on the removable sill but uncured foam in the centre. This 'bubble' can crack in weeks time, or in a years time and start to cure again, pushing your sill off and leaving a huge bulge.

2/. What happens if you damge your sill? It will prove extremely difficult to remove it after you have effetively glued it onto the car.
 
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