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Which sealant to use on rear bumper seam?


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I have decided to seal the small gap between my Rapido's rear bumper (ABS) and the aluminium body panels to stop the water getting down through the narrow gap. My reason for wanting to do this is a short while back I found that some of this water was finding its way past the inner sealant and into the rear garage. I have sorted the ingress problem out but now want to completely divert the water at the point where it enters the rear-of-bumper cavity. The bumper is white so I will need to use a sealant of the same colour but also one that will do the job WITHOUT permanently sticking the walls of the 'van and the bumper together!

When repairing the old black inner sealant that needed to be cut away to check for any interior damage, I used black Sikaflex 221 - excelent stuff but that, of course, is also a strong adhesive.

Can anyone offer/recommend a Sikaflex or any other product (other than normal silicone bathroom sealant)?




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Sikaflex 512 is a polyurethane adhesive sealant designed for construction and repair of caravans, I wouldn't use a silicon as they have limited expansion only 10-20% depending on the product. Anything you use if it's going to keep the water out is going to need to bond to the body & bumper just have to get your Stanley knife out if you want to separate them.
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Hi Dick,


It sounds like you need...(Quote from CAK Tanks website, page 115)... Link to CAK Tanks




Sikalastomer 710 butyl sealant is a high quality

non setting sealant used for a variety of joints

in caravans and motorhomes. It is especially

suitable for those situations where there is the

possibility of future disassembly, such as edge

trim on motorhomes, caravans and boats,

windows, rooflights and airvents etc. Due to its

very sticky non-setting flexibility it is ideal for resealing

of edge trims that have been sealed

with ordinary non-setting mastics which dry out.

Available in black, grey & white, size 300ml







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I think it may be difficult to get a good-looking seal with SikaLastomer-710. Butyl sealants are horribly sticky and this product is designed for tasks like waterproofing the rubber gaskets of classis-car windscreens. I note from SIKA's technical data sheet that it has a 12% shrinkage rate and a 12 months shelf-life (check the date on the cartridge). Asking-prices vary considerably.


I'd probably opt for Sikaflex-512 (mentioned by lennyhb) or, if a faster cure is required, Sikaflex-221.


I can't see gocro's 'vaseline' idea working in practice. Whatever sealant is used (as lennyhb advises) I believe it will need to adhere to the bumper and the body panel, not just to waterproof but also to enable a tidy joint to be achieved. If that means that the joint has to be cut through at some later date, well that's tough luck.

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I wouldn't shell out for sikaflex for a job like that. Go to b&q or similar and get a non silicone sealant for half the price. You don't need a high performance adhesive like sikaflex.
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I have a similar problem at the moment. It has been caused by reversing my van on to my drive. This means water can pool on the rear bumper due to the slope.


My van is a Burstner and it looks to be a difficult job to remove the rear bumper but it will have to be done by someone at some point. Luckily it is localised to one corner of the garage only. I have turned the van round but it is a pain to manouvre on to the drive this way. :-(


I cannot do any more until it dries out a bit.


The question I ask is this. Are these bumpers originally sealed to the bodywork? It looks not on mine and there is a strip of sealant applied underneath where the floor meets the bodywork. It is not a huge amount of sealant and has lost contact with the wood due to the damp and will need renewing.

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From what I have seen, there is no consistent approach to this. Some fit the rear "bumper" (but don't bump it, 'cos it will just break! :-)) to lie against the rear panel, some add a soft cap to the edge of the bumper moulding to prevent it chafing against the panel, some seem to seal the two together, and some have the bumper moulded as part of the rear panel. Ho hum!


For those where the bumper is planted on, the general approach seems to be that integrity against water ingress is provided by the panel joints, rear to floor and rear to walls. So, if those joints remain sound, water by passing the bumper and running down the bottom of the rear panel should not be a problem.


I'm guessing a bit, but from what you say, it sounds as though the problem is mainly with the floor to rear panel joint, rather than with the lack of seal between bumper and rear panel. I wonder if you may have reversed over some rough scrub that has torn at what sounds like a surface applied seal, presumably not intended to be seen. The problem with sealing the gap is that the open joint may be intended to allow the area behind the bumper to breathe somewhat, which a sealant would negate. It is a very exposed and mucky area when driving on wet roads, so any moisture drawn up behind the bumper would not then dry as effectively as with no seal. Also, if it is the floor to rear panel joint that is in trouble, sealing the bumper would not help with the problem of spray etc when driving and will still leave that joint exposed and wet.


I think it is the floor to rear panel joint that needs treating and, if that is done, whether you seal the bumper to panel joint is probably not of great significance. Until you can get the floor panel thoroughly dry to be able to deal with it, could you tape a strip of polythene across the rear of the van and onto the bumper, so that any further rain is kept at bay until the weather improves?

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In my case Brian I think it might be a breakdown of the sealant where the garage floor butts up to the rear panel. It is open to spray from wet roads and also susceptible to water draining off the bumper towards the back panel (due to being parked on a slope). Any kind of extra protection does not look easy. Once dry, removal of the old sealant and application of new is necessary as a minimum.


In reality, I will have it checked by a local company and see what they recommend..

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