Primer then filler or other way round

[rogerowen]

I have a few rust holes in front valance. I've Mig'd in some backing metal behind the holes and I'm about to apply a thin coat of filler. Question is should I prime first then fill - or fill first then prime?

[mogbob]

I'd treat with some rust preventative first,allow to dry well, filler then primer.
I assume that you've "dressed " the welds on the unseen side already ? If not, do that first, otherwise the heat from the angle grinder disc / flap wheel might crack the filler and you'd be back to square one.
Re cap ..Prep first, metal down to flat surface,treat against rust, clean then filler, then primer.

I personally go for butt welding patches in.Any horizontal surface will more susceptible to rust because water will lie on the surface rather than run off.
Bob

[ossiemobile]

In GRP boatbuilding, we inserted mild steel reinforcement plates in the laminate, then a year or two later would discover them madly rusting inside the grp...... filler is same stuff [polyester resin & filler powder] so now you KNOW that filler paste on bear steel WILL corrode. BEST answer was to zinc plate the steel, so I would look for a brush or spray applied zinc rich "paint". Phosphating also worked well, but at home how would one cold apply this ?? & why I'm here, we also found resin detaching from inserted plywood pads....caused by differing expansion & contraction rates ..... so filler straight onto WOOD is bad idea as well [eg Traveller botching...

[tysonn]

I've always put filler on bare metal.Could be wrong but I'm pretty sure that's what the manufacturers recommend.Don't remember having rust problems later on.
Mick

[kennatt]

As above, if you prime first ,the filler can not bind to the metel, the idea being that you grind /sand the bare metal to provide a rough surface to apply the filler(As per manufactures rec.). However you must get rid of all the rust by grinding. If you apply primer or any coating to the metal. The filler will not bind,and when flatting its impossible to feather edge the filler,since there is a layer of primer/coating under neath and this simply lifts off the metal at the edge of the filled area,and when you try to prime over this area you get sinkage and a ring round the filled area.
Glassfibre bodies like boats suffer from osmosis ,thats how the water gets to the metal reinforcements,polyester and epoxy resin is inert once cured,metal need water to rust.
If you were and old knacker like me you would lead load the area,but thats another story and skill slowly dying out

[rogerowen]

As above, if you prime first ,the filler can not bind to the metel, the idea being that you grind /sand the bare metal to provide a rough surface to apply the filler(As per manufactures rec.). However you must get rid of all the rust by grinding. If you apply primer or any coating to the metal. The filler will not bind,and when flatting its impossible to feather edge the filler,since there is a layer of primer/coating under neath and this simply lifts off the metal at the edge of the filled area,and when you try to prime over this area you get sinkage and a ring round the filled area.
Glassfibre bodies like boats suffer from osmosis ,thats how the water gets to the metal reinforcements,polyester and epoxy resin is inert once cured,metal need water to rust.
If you were and old knacker like me you would lead load the area,but thats another story and skill slowly dying out

Thanks - that's sound advice. Yes, I'd love to have a go at lead lodeing, kit looks a tad expensive from Frost though.

[tysonn]

Lead loading is dead easy................on a horizontal surface!Anything else make sure you have something on the floor to collect what runs off to re-use if you're doing it on a vertical panel.Use solder paste to tin it first.Very satisfying though when you get it right.I used it a lot on my RM Rileys.
Mick

[rogerowen]

Lead loading is dead easy................on a horizontal surface!Anything else make sure you have something on the floor to collect what runs off to re-use if you're doing it on a vertical panel.Use solder paste to tin it first.Very satisfying though when you get it right.I used it a lot on my RM Rileys.
Mick

In danger of going off topic - but hey! There's a simple method within the MM original workshop manual. File size probably a bit to big to upload here - so I've posted it on my DropBox, you can see it here;

https://www.dropbox.com/sh/b6oll8ce8jbm ... CWrTa?dl=0

[kennatt]

Lead loading is dead easy................on a horizontal surface!Anything else make sure you have something on the floor to collect what runs off to re-use if you're doing it on a vertical panel.Use solder paste to tin it first.Very satisfying though when you get it right.I used it a lot on my RM Rileys.
Mick

Ahh you've let the cat out of the bag Mick, don't let the majority know about those mystical skills hidden in the mists of time.Then they will be as wise as we are(and Roy of course) and that would never do

[Mark Wilson]

As I'll hopefully be moving on to panel prep fairly soon, has anyone any advice on favourite brands of fillers, or any to avoid?

Mark

[tysonn]

Subjective of course.You can make the job a dream or an absolute ba***rd.In my opinion the absolute best by miles is U-POL fantastic.Its superfine,no pinholes,a dream to flat etc etc.If you can't find it jawel-paint on Ebay(amongst many others)sell it.
Mick

[Mark Wilson]

Subjective or not, you do seem to know what you are talking about, so thanks, that's good enough for me.. When you were honing your lead loading skills all those years ago, I was jamming Isopon and chicken wire into rusty holes........

Mark

[tysonn]

Well I must admit I did some of that too.And quick partial resprays from aerosols in the carpark of Rothwell Motor Auctions as the driver drove them through.Nice gloss that wet paint.
Mick

[rogerowen]

'I was jamming Isopon and chicken wire into rusty holes........ '

Wot? Chicken wire instead of scrunched up newspaper! Dead posh!

[kennatt]

'I was jamming Isopon and chicken wire into rusty holes........ '

Wot? Chicken wire instead of scrunched up newspaper! Dead posh!

Allmost as good as a wooden sill I once made up to fit my Hillman minx, or the tate and lyle printing on the metal of a chassis repair I discovered when I bought an HB viva, Don't know how he had done it but it was actually spot welded on,must have been a good welder cos it was as thin as fag paper.