I recently welded in a large patch at the back of the cab floor on my 53 F-100. I thought I was being pretty careful, but now I wonder if I have inadvertently created a problem. It is my intention to install the angle piece (long ago rusted away) that was under the seat and which formed the storage place for the jack. I assumed that the floor section onto which this piece is attached is flat. It turns out that I now have a small crown in the floor at that location. I know that the main floor is supposed to have a crown, but at the very back of the cab, the floor is flat where it is reinforced by a stiffener. There is a small step in the floor pan level where it would appear that the crowned portion of the floor meets the flat portion of the floor. My problem is that I have a crown on both sides of this step. Is that rearmost section of floor supposed to be flat, or does it have a shallow crown? My attached drawing may create even more confusion than this post, but if anyone has their seat out, maybe it can be determined whether my cab is okay, or that I created a condition that I have to deal with. Thanks.

 

Welding in a panel, especially a flat one, can make it crown up in the center. The welding shrinks the metal along the weld beads, which makes the center of the panel larger than the edges, so the only place that excess metal can go is to hump up (pillow). The best way to remedy this is to prevent/minimize it in the first place by using a small diameter welding wire (assuming you are using shielded MIG?) as close to the metal thickness as possible. I recommend 0.025 ESAB easy grind wire for sheetmetal work. Limit your welds to tacks no more than 1/2" long, no closer than 6" apart. Allow each set of tacks to cool COMPLETELY until you can put your bare hand on top the weld and leave it there before the next set of tacks 1/2 way between the previous set. Don't rush! Walk away and allow the panel to cool completely to room temp if the center begins to warm in the least. Continue this technique of 1/2" beads no closer than 6" and then cool completely (DON'T force cool with water or compressed air!) until the panel is completely welded. Grind the weld bead near flat being VERY careful not to over heat the area with grinding or else you will just defeat everything you have done. If you can't put your bare hand on the ground area within a minute, you overheated it. I use a 40 grit red fiber sanding disk with rubber backer on my 4" angle grinder. The disk should be flat to the panel and only contact the weld bead. Use a fresh disk and a very light touch, don't try to get it flat in one pass! If the metal colors you got it too hot!
If you did exactly what I just described the pillowing should be minimal to none. If there is pillowing, it may be eliminated one of two ways: 1. Stretch the weld seam using a steel dolly and a low crown body hammer. Push the dolly hard against the back of the weld seam (You did grind the back side near flat as well as the front side?) so the metal bows up. Tap the seam with the hammer once directly above the dolly (you should get a ringing sound not a thud). Tap once again slightly further along the seam but always over the dolly so the hammer marks overlap about 1/3 their diameter. Continue this until you have gone all the way around the panel, being sure you are pushing hard with the dolly and each tap is on dolly and equal in strength to the others. Check the panel for flatness with the edge of a straightedge across the weld seam. Repeat as necessary until panel is flat.
2. heat shrink the center of the pillow. Often you may not be able to access both sides of the full seam to stretch it. In that case you will need to heat shrink the center. Do an advanced search on this forum, my user name and heat shrink for my post on doing a heat shrink for a complete explanation and link to my diagrams.

 

I was pretty careful patching the panel in. The small crown that I see is along the line of very short tacks where the panels butt. The reinforcing stiffener piece is less than four inches away from this 36 inch long butt weld, and at that point, the floor is flat. I spent some time looking around online for photos of cabs that still had that piece of angle steel which forms the jack storage area. I found a few. I was surprised to see that the piece was obviously stamped, not simply folded on a brake. And, maybe it's just me seeing what I want to see, but it looks like that angle piece is stamped so that I fits over a crowned surface. At any rate, what I have now is not visually distracting. The crown will do what it's supposed to do: direct any water out of the cab. And I will see how good my metal shaping skills are when I try forming a bend piece of steel to neatly fit over the crowned floor.