Synthetic potting compound

[SurreyNick]

Can anyone recommend a decent and affordable synthetic potting compound? I want it to re-stuff old capacitors and resistors into which I have substituted new components. These vintage components were originally stuffed with black pitch, so I want something that has a similar appearance and properties. In some cases e.g. with the surface mount condensers the pitch is visible.

I don't want to use wax. I have read somewhere that black silicone sealant works but I am concerned this dries to a shiny gloss and I am not sure the original pitch would have had the same appearance. On the plus side it's cheap. The downside for the synthetic potting compounds on RS Components is that it's nearly £30 for 250g! The other sites I have found don't even quote a price - not a good sign.

Can anyone recommend an affordable alternative and vendor?

Thanks

Nick

[David G4EBT]

I can recommend a two-part resin filler known as 'Super Steel'.

Right away you may feel that the term 'steel' renders it unsuitable as steel is a conductor. I think it's just a marketing term - there was a time when the adverts for 'Ir'n Brew' said 'Made from girders, in Scotland'. Just a bit of ('scuse the pun) 'irony' rather than iron, most people realised that, but the Advertising Standards Agency put a stop to it as it wasn't a 'truthful' claim. Whether or not Super Steel does have any steel in suspension, I know not, but before I started using it, I put some on a strip of glass 1cm wide x 4cms long and applied my Victor VC60B Insulation tester across it on the 1000V range. Not a trace of conductivity.

As an aside, I use Super Steel a lot for other things such as Bakelite cabinet and knob repairs, re-fixing valve bases, filling holes on pre-loved die cast alloy project boxes etc - far superior to Araldite. Sets rock hard in minutes and you can drill, file or sand to a smooth finish, but that's by the by. When it sets, it's very dark grey and shiny, just like pitch. When worked, it's a lighter grey and dull.

I've attached some pics, the first of which is of some caps I've stuffed for a pre-war Ecko 'AC77' that I'm (slowly) restoring, the second shows a cap end on, and the last one shows a strip of Super Steel on glass being subjected to 1,000V insulation test. The last pic shows some of the original caps in place before I totally stripped the chassis and set about restoring it. There's no sense to it of course - it's not a valuable set, but I decided that just for once, I'd pull a radio to pieces and do a full restoration rather than just change a few caps, resistors and valves. Stops them putting me in a home

For the avoidance of doubt, the '1' on the display of the V60B doesn't mean 1 M Ohm - it means infinity.

(Super Steel is widely available on 'tinternet).

Hope that's of interest.

[Leon Crampin]

Don't pot re-stuffed capacitors. They will become stressed due to unmatched temperature coefficients of linear expansion and there may also be chemical compatibility problems with some capacitor coatings.

I did a lot of work on automotive systems design and potting is a minefield which should always be avoided unless absolutely essential. The environmental conditions relating to a new capacitor in an old cardboard tube fitted inside a radio are hardly onerous and they will live quite happily in the tube suspended by the leads.

By all means plug the ends to give an authentic appearance, but be very careful with some silicone rubber compounds. The acetic acid used to aid curing will corrode the leads - usually inside the tube where the vapour is trapped. I usually use a plug of stiff wax, coloured as appropriate.

Leon.

[SurreyNick]

Mmm. Super Steel sounds ideal David, but I'm concerned about your post Leon. I'm not worried about the resistors but the surface mounted capacitors are another matter. Size-wise they will happily accommodate a ceramic disc capacitor as 0.1uF is the max capacity. But do you think these will suffer as you describe when potted with Super Steel? The whole side of the housing has to be stuffed as shown in the picture.
Nick

PS: Very impressive work David

[Ed_Dinning]

Hi Nick, you can always use pitch, see who in your neighbourhood is having a flat roof fixed and cadge a piece from the roofer. It stays quite soft so should not stress parts too much.

Ed

[SurreyNick]

Blast. Had my flat roof replaced 2 months ago!
Oh. well.... I'll keep my eyes open.
Nick.

[David G4EBT]

Just to clarify my earlier post, I didn't totally encapsulate the caps inside the paper tubes - I plugged the ends with kitchen roll paper, and finished them off with about 6mm depth of 'Super Steel'.

Your right Leon about acetic acid in commonly available silicone sealant as sold by DIY stores, often termed 'sanitary sealant' for sealing around baths, toilets, worktops etc. Some years ago when I was still active on amateur radio I acquired a second-hand mini-beam in which the wire elements had been terminated in an ABS box in a terminal strip. The whole thing had been covered inside the box with silicone sealant to protect it from water ingress, but it was shot to bits with corrosion brought on by the acetic acid - the multi-strand wires were green with corrosion and almost rotted through. I re-built it with a new box, terminal strip, wire elements, and sealed it with a couple of coats of yacht varnish instead of silicone sealant.

Three years ago when I dismantled and sold my telescopic mast and all my aerials, I took it down after about ten years at the top of the mast, took the lid off and it was as good as new inside.

[Framer Dave]

Originally Posted by Ed_Dinning

Hi Nick, you can always use pitch

Ed is right. I've recently been making several replica batteries and I've sealed them with pitch (more often called "Bitumen" in the trade these days). Trouble is,trade quantities are typically of the order of 25kg (a lot!) and the cost seems high relatively. Browsing the Internet I found this:

http://www.ebay.co.uk/itm/BRAND-NEW-...item5668620f7b

This cost about £9.00 post free and will do a lot of capacitors, batteries etc. Do be careful with it though, its melting point is high and a splash on the skin will stick and cause a very nasty burn. If you overheat it and set it alight, just back away DON'T USE WATER.

Attached a picture of a recent battery, sealed with this stuff.

Regards,

[G6Tanuki]

Can you use soft(ish) paraffin-wax? That will provide support for the entrained capacitor and help exclude moisture, but will remain suffifiantly flexible that it won't mechanically stress things.

[Must admit, whrn I've done 'potting' it's always been aimed at surviving immersion, pressure-cycling and vibration/maximum-G-forces - there are a range of resins and non-acidic silicones available for such applications but you also need to consider how the components are mounted - a single-turn loop formed on resistor- and capacitor-connecting leads is commonplace even in 1980s-era car electronic-ignition/injection systems]

[Diabolical Artificer]

I think this is an good use of the wax covers that you get on some cheeses. I've a bag full waiting for a use ; they are red though. For pitch or bitumin , if a small amount is needed, then look on the road. You sometimes see little pools of it where it has oozed out. There is pine resin as well, which of course is free too, and melts at low temperature. Lastly you could use a black plastic food tray, melted with a heat gun.

Leon has a point though, especially as some types of ceramic capacitors vary wildly with temperature, as much by as much as a a 1000% off value in some classes. See Wikipedia.

Andy.

[Radio Wrangler]

Hot-melt glue gun? the 'ammunition' seems to be straight forward polyethylene which is an excellent dielectric.

G3ROO put me onto this stuff as a sealant to coat outdoor antenna connectors.

David

[Radio_Dave]

Originally Posted by SurreyNick

I don't want to use wax.

What's wrong with wax? You could melt black wax crayons.

David

[SurreyNick]

A wonderful and comprehensive array of options. From the very simple to the permanent. My thanks to all who posted
I came across one other of my own too - Milliput Epoxy Putty. I think it's a bit like Super Steel insofar as it's a two-part epoxy. Comes in black and other colours too.
Nick