PVA and lead shot reaction/expansion problem

[Atlantic 3279]

I believe that the problem of lead shot ballast, PVA glued into confined spaces in loco bodies, and then expanding and splitting the loco structure, has been aired to some extent in the letters section of Model Railway Journal. Not being a regular reader I've missed out on any conclusions that have been reached regarding causes, contributory factors, and remedies for the problem (if any). I wonder if any contributors to this forum have any pearls of wisdom on this subject?

I know a couple of modellers whose locos have been affected by this problem, but I do not know how long the trouble has taken to appear and what particular type of PVA adhesive they have used - if the exact type makes any difference.

In view of the possibility of trouble, I've just inspected the only loco in my collection which has a boiler-full of PVA glued lead shot, completed almost 4 years ago, and I cannot see any hint of bulging or cracking. I'm not feeling complacent yet, but I'm just wondering whether the reaction problem stems from re-absorption of atmospheric moisture after intial drying of the glue, and whether I may be lucky in as much as I used "weatherproof" PVA, which presumably shows less absorption of (or reaction with) moisture once it has "gone off", AND my lead shot in this case is also fully sealed off from the atmosphere by a covering of (polyester?) resin of the kind used in glass-fibre construction.

[mick b]

Seen this mentioned a few times including photos of major damage normally to smokeboxes. No idea of variation of type of PVA glues etc. I would recomend get it out if poss ASAP

Mick

[chaz harrison]

I have personal experience of this problem. Some years ago I built a 7mm model of a GNR saddle tank from an etched brass kit. I partially filled the saddle tank with "Liquid Lead" bonded with pva, both fed (appropriately) through the hole for the tank filler casting. The loco ran on my layout for a few years without a hint of the problem. One day I was cleaning its wheels and giving it a good dusting when I noticed a white line at the front of the saddle tank. I assumed that this was caused by flux residues attacking the solder joint so I scraped it and retouched the weathered black. A couple of weeks later the problem re-appeared. Over the next couple of months or so a gap opened up as the expanding ballast pushed the tank front out. Eventually the gap was over a millimetre wide.

I did a partial dismantle of the loco using a gas torch. I then stuck the tank in a pot of water for a couple of weeks. The lead could then be poked out with a screwdriver and the body rebuilt.

I did take the chance to correct an error on the tank handrails and also, more significantly, to rebuild the frames from rigid to fully compensated, so there was a silver lining.

My advice to you would be to remove the lead if you can without a total dismantle. If removal would require a total disassemble I might wait until it obviously becomes necessary - what have you got to lose?

Chaz

[Atlantic 3279]

O 'eck! It looks like I'd better do something about it then. Fortunately the skin of polyester resin in my case is across the large void in the bottom of the boiler, which DJH left open 'cos they want you to put the motor in that position - I didn't. It may be a fairly simple matter to drill into the "cake" of ballast in several places and then dig/scrape the bulk of it out. Maybe I can then refill the void with dry lead shot and poor polyester resin alone over this to keep it in place. As this resin is definitely waterproof when cured, has no water in it to start with to initiate any long term reaction, and doesn't produce acetic acid when going off (which I believe PVA does) I cannot immediately imagine that any reaction is likely to occur.

[jwealleans]

I'm not sure I'd rush into it, Graeme; assuming yours is 4mm, it seems to me that most of the MRJ examples, like our friend Chaz's above, were 7mm. It may be that the larger volumes involved (with commensurately longer drying times, one assumes) are part of the equation.

I have one of two locos I've weighted with lead strips and then shot poured into the gaps, but I'm leaving those well alone to see whether anything untoward occurs.

[chaz harrison]

Well 3279,

I think Jwealleans is right to counsel caution. Leave well alone if you don't have the problem.

I'd certainly be a bit careful with that drill, I found that the Lead shot/PVA mass resembled concrete - very hard, and certainly much harder than either brass or white metal. It resisted my attempts to scrape it out when I attacked it initially. If you are going to try and remove it, then you will need to break through the polyester resin. I would then leave the mass to soak in water for a while. The PVA will soften and make removal a lot easier.

As regards avoiding this problem in the first place I have stopped using "Liquid Lead". I now use sheet lead, rolled or folded up, to fill the available space, fixed in with tape, Plasticene, UHU etc but I avoid letting PVA get anywhere near lead. I think the oxides that form on the surface of the lead are the cause of the expansion, and water retained in the set PVA causes the corrosion. Of course if you are filling a totally enclosed volume (like the saddle tank on a J52!) you could pour in lead sheet and then seal it, with no need to form a solid mass.

Chaz

[Atlantic 3279]

Thanks again to both of you for the comments. I take it that you mean shot rather than sheet in that last sentence Chaz. It had occurred to me after posting my last message that maybe I ought to check again to see what kind of "bulkheads" were forming the front and rear ends of my lead mass in the boiler. I can't remember whether I filled all the way to front of the smokebox (I may have partitioned some sapce off with a disc of plastic) and at the rear I certainly partitioned off the firebox which houses the motor. With luck, my installed bulkheads, or the resin layer over the bottom slot in the boiler, will bulge or burst if any expansion has to be accommodated, leaving any externally visible soldered joints undamaged.

Graeme

[52D]

Chaz you need to change your name to Dai Woodham, thats a great pic of the pannier. No im only kidding its GNR not gods worst railway.

[chaz harrison]

Chaz you need to change your name to Dai Woodham, thats a great pic of the pannier.

Dai Woodham? No, I think if I am going for a name change then it will have to be Captain Smith, as the saddle tank is now back on the layout, and running beautifully on its new compensation.

...and yes 3279, I muddled my sheet with my shot!

If, as you say, you have "soft" bulkheads either side of the lead mass then you may well get away with it. The expansion is inexorable and can't be resisted but a soft material might well absorb it.

[Autocar Publicity]

I think a similar thing has happened to me recently, though with rolling stock. I added Liquid Lead to a Dapol van which I had worked on and (several months later) last Sunday was rather disturbed to notice that one axle wasn't revolving. Upon investigation, the main weight as supplied by Dapol had been forced out of position and the plastic chassis was bowing. The extra lead was removed in double quick time and the chassis has now mostly recovered though is still very slightly bowed.

I have also glued lead into [smaller] parts/crevices of a couple of Colin Ashby's OO9 carriages to add weight, so far these have not reacted, fingers crossed...

[Atlantic 3279]

Thanks for another response. Please keep 'em coming, as enough contributions from people with differing experiences may help to show what are the relevant factors that lead to an actual problem being experienced.