Expanding foam ?

[RichardHyde]

Tonight’s question…is it a good idea to fill the sills with that expanding foam ? If it completely fills the sills, then would it stop water getting in and future rust ?

Thanks, Richard

[JerryTR5]

Probably not. The foam is full of air pockets so it will still get water & oxygen entrapped. It will make the sills immensely strong though.
On the downside, when you do need to weld it, should go up a treat
Jerry

[cliftyhanger]

Best way to prevent rust is decent quality rustproofing. That means a quality, thin wax prayed in at a decent pressure, and best on a hot day so it creeps nicely into the seams.

I used Dinitrol MIL stuff on my spitfire 7 years or so ago, most is still good but a couple of areas which I didn't wax have some bubbles.
Dynax S50 in an aerosol is dead handy, and I have just boiught 5l each of dynax S50 and UB. Yes, I do have too many cars....

[new to this]

No do not fill the sills with expanding foam, bad idea

Dave

[ian.stewart]

Expanding foam, NO, Expanding foam is set by using moisture in the atmosphere, if you want to make expanding foam set faster wet the surfaces first. so I would guess the foam is Hydroscopic in some form, and probably retains a modicum of moisture,
A few years ago, Myself included, in good faith, when rebuilding cars would seal seams with bathroom sealant, this was even advocated by several "Classic Car" mags as a good sealant. the trouble with bathroom silicon, its setting agent is acetic acid, (it also acts as a etch for adhesion to ceramics,) it got to a point where you could buy cars that looked really good, until looking deeper, you spotted rust by a seam, pull at the sealant and you could virtually unzip repairs where the acetic acid had done its worst.
I dont think Expanding foam would be much better,

[xvivalve]

Expanding foam, NO, Expanding foam is set by using moisture in the atmosphere, if you want to make expanding foam set faster wet the surfaces first. so I would guess the foam is Hydroscopic in some form, and probably retains a modicum of moisture,
A few years ago, Myself included, in good faith, when rebuilding cars would seal seams with bathroom sealant, this was even advocated by several "Classic Car" mags as a good sealant. the trouble with bathroom silicon, its setting agent is acetic acid, (it also acts as a etch for adhesion to ceramics,) it got to a point where you could buy cars that looked really good, until looking deeper, you spotted rust by a seam, pull at the sealant and you could virtually unzip repairs where the acetic acid had done its worst.
I dont think Expanding foam would be much better,

I took one car apart almost entirely using this method!

[marshman]

Early BMC Minis had foam filled inner sills. This was done at the factory to stop the ingress of water. Anecdotal evidence of people that have restored such cars say they tend to have less corrosion than non foam filled cars.

Having said that I wouldn't do it. Once done it is incredibly difficult to remove, which you would need to do to carry out any subsequent work in that area. Also not easy to inject and ensure you fill all the cavities - very difficult to assess the quantity required. 2 part PU foam usually has an Isocyanate based catalyst and "out gasses" for some considerable time. The foam it forms is of a "closed cell" nature so is supposed to be "waterproof". However prolonged immersion in water eventually results in it becoming saturated, takes about 10 days of constant immersion - this is from my own research done at my old work where we used 2 part PU foam as a filler in one of our products. If you did do it one by product might be increased stiffness of the sill sections, assuming the foam bonds properly to the surface. Wetting the surfaces does speed up the cure time and aid bonding but is not absolutely necessary, we didn't do it on our product, but we did have to leave it for 3 days to fully cure..