Run your hand over some of your lower panels and you are bound to find rust breaking through. You might incur some measly bumps from old filler. Or it might just be rust breaking to the surface. Whatever the case, it needs to be sorted. If you don't fix it, it will just get bigger and grow worse.
Treating bubbly rust from the outside is a complete waste of time and money. In almost all cases the rust is on the inside of the panel, and like an iceberg, only 10 per cent has broken through.
How to tell if it's New or Repaired Rust
Smooth round bumps usually mean that the rust has been repaired before and filled with either plastic filler, or lead. The rust has continued to form, and push the filler out, (when metal rusts, it expands to many times its original size.) A dig with a sharp screwdriver will usually reveal a pinkish filler or shiny lead.
Small irregular bumps, with some brown stain, usually indicate new rust. A firm push with a narrow screwdriver will normally poke a hole right through the panel. This is called perforation rust.
In body crevices, or where paint is thin, surface rust can form. This can be identified by brown scale. Unless it is very severe, a screwdriver will not penetrate. Surface rust is not normally serious. All that is required is sanding back to shiny metal, and repainting.
Repairing Perforation Rust
• Preparing for filling
You will need the following:
1. Cork sanding block, approx 125mm x 75mm
2. 80 grit Drilube sandpaper
3. A carpenter's hammer
4. 1 x 100mm nail
Sand level any old filler that has become lumpy. If it has cracked or appears unstable, dig it out with a screwdriver or similar. If the remaining filler appears solid and well anchored, leave it as is.
Using the nail and hammer, punch in all of the rotted metal as indicated by small irregular bubbles. Look carefully all around the affected areas for bubbles about to form. These will show up as small brown stains. Punch these also. You should now have a series of round holes.
If there is more extensive rust that has already formed holes, use one of the claw tips of the hammer, to punch the rotted metal at least 6mm inward, around the hole. Always ensure that there is no weak metal level with the surface to be filled. This is best done by again tapping all around the area with the claw tip of the hammer. Finally, roughen the entire area thoroughly with 80 grit sandpaper, using the cork block. Blow off the excess dust and make absolutely sure that the area to be repaired is dry, inside and out.
Where possible, in the case of holes larger than 25mm in diameter, it will save time and filler to wedge a piece of cardboard, or similar, on the inside of the hole to support the filler as it dries. A ball of newspaper can be used to hold the cardboard in place. This can be removed later.
