Rust around mushroom vents - do they need to be removed?

[Lizzy]

Generally the paint on my roof is in good nick, except bubbles of rust that have formed around the mushroom vents over the winter. I have removed the rust with a wire brush and sand paper, but I'm wonderning if I need to remove the vents to clean out any rust that may be underneath? The vents are not leaking at all, so I hesitate to open up a whole new can of worms there by removing them. I don't know why the rust only happens around there. I can't see any rust anymore with the vents on.

I was thinking to put on a primer, then glossy paint (same as the rest of the roof). Is that enough?

Also removing the rust slightly damaged the sealant around the vents in some places. I was thinking to repair it with silicone made for outside use. Do I want to put that on top of all the paint?

Sorry these may be daft questions, but this is the first time I'm doing this so would appreciate any advice.

[matty40s]

You will find that some painters do not remove windows and mushrooms etc when doing a full repaint. 

This allows moisture to get between the old layers that remain and the new paint, thus resulting in rust. 

Yes, it is better to remove the mushroom completely, remove rust, paint and then reseal the mushroom with tape(as discussed in the removing resealing windows thread recently)

[WotEver]

Alternatively, leave the mushrooms in place, treat the de-rusted area with Vactan, followed by primer, followed by top coat. Ensure that all 3 get into the gap between the vent and the roof where you've damaged the sealant  

Do not use Silicone sealant. 

Edited March 31, 2017 by WotEver
Reworded for clarity

[Murflynn]

as matty says.  .........................   and I would advise you use some rustkiller on the exposed metal before priming.  Vactan is good, but there are similar products available at a good DIY store.

 

crossed with wotever.

trouble with vactan is that there is AFAIK only one supplier, mail order.

Edited March 30, 2017 by Murflynn

[Lizzy]

Hm OK, sounds like a bigger job than expected. I am not sure this is all feasible at this time of year with so many coats and removing the mushrooms, but I'll try

Could somebody point me to the thread (or the product) of the tape that I should use? Thanks ever so much.

[rusty69]

 

[rusty69]

36 minutes ago, Murflynn said:

as matty says.  .........................   and I would advise you use some rustkiller on the exposed metal before priming.  Vactan is good, but there are similar products available at a good DIY store.

 

crossed with wotever.

trouble with vactan is that there is AFAIK only one supplier, mail order.

Looks like you can buy Vatcan on ebay click and collect Argos

[roland elsdon]

Before you start attacking the screws find out how the vents are fastened if it's retaining nuts unsecured and inaccessible then the nuts will fall behind the lining and you will have nothing to screw to. Alternatively you may find vents sealed down with silicone and screws pushed in on a bed of silicone with no nut. Whatever happens you have to be able to get at the nut as well as the screw.

[Boat&Bikes]

I would go with wotevers advice. Sometimes things can be overthought and overdone. Ian .

[matty40s]

2 minutes ago, ianali said:

I would go with wotevers advice. Sometimes things can be overthought and overdone. Ian .

.....and do the same again to them every Winter until you finally decide that enough is enough and whip them off and do it properly.

[Boat&Bikes]

2 minutes ago, matty40s said:

.....and do the same again to them every Winter until you finally decide that enough is enough and whip them off and do it properly.

Or find three years later that all is well.

[Cloudinspector]

30 minutes ago, ianali said:

Or find three years later that all is well.

If I was a betting man I'd wager on the rust returning within 3 years.

[Boat&Bikes]

6 minutes ago, Cloudinspector said:

If I was a betting man I'd wager on the rust returning within 3 years.

Quite possibly. I wasn't suggesting it's the perfect solution but worth a try. Removing well sealed vents might lead to problems with leaking. The sun is out there are better ways to spend your days. Ian .

[matty40s]

I've probably painted 30 boat roofs in the last 3 years, if there was rust round the mushroom,,there was rust under it, every time.

The area around the vent us being done anyway, not much extra time to do under the vent too.

Edited March 31, 2017 by matty40s

[Murflynn]

6 hours ago, roland elsdon said:

Before you start attacking the screws find out how the vents are fastened if it's retaining nuts unsecured and inaccessible then the nuts will fall behind the lining and you will have nothing to screw to. Alternatively you may find vents sealed down with silicone and screws pushed in on a bed of silicone with no nut. Whatever happens you have to be able to get at the nut as well as the screw.

I found it was worth while to tap a thread in the existing hole, and refit the vent with the next size larger machine screw.  Easy to replace by buying on ebay (Kayfast is the first vendor of stainless bolts that comes up).

[Cloudinspector]

As an owner of classic cars for many years I can guarantee that what appears to be a small rust spot will be much worse once you start investigations. 

[alan_fincher]

Matty and Cloud Inspector are correct.

Anything that involves not removing are at best a bodge.  There will definitely be rust under the vent, and it is likely the steel of the roof will be eroded far worse under the vent than around it.

Delaying doing a proper fix, means that when you finally do tackle the inevitable, the accumulated damage will have likely got worse.

(I should have taken pictures of what was under the one I took off about a week ago - it didn't look to bad until I got under it, and it wasn't yet actually leaking at all.)

Edited March 31, 2017 by alan_fincher
typo

[dmr]

31 minutes ago, Cloudinspector said:

As an owner of classic cars for many years I can guarantee that what appears to be a small rust spot will be much worse once you start investigations. 

Boats are actually much better than cars, thicker and better steel.

The rule of thumb with old cars is that the rust is always at least three times worse than it looks, but with boats its usually less than twice as bad. 

............Dave

[matty40s]

 

Always expect the unexpected. ..

Boat comes in, wants the shower resealing as they keep finding water in the dry bilge.

Check piping, ok check calorifier, ok, check water tank..ok.

Lift locker lid...

 

In front of the front doors on the well deck is a v channel leading to drain holes eacside.drain holes less than an inch above canal height .every time someone steps on side of boat, water comes up channel.......and drops through rusted hole into bilge..this was hidden under a metal plate and lots of rubbish in locker.

[Horace42]

2 hours ago, Murflynn said:

I found it was worth while to tap a thread in the existing hole, and refit the vent with the next size larger machine screw.  Easy to replace by buying on ebay (Kayfast is the first vendor of stainless bolts that comes up).

Yes! drill and tap holes and use SS machine screws. That's what I did with mine (flat dish type). So it makes it easy to do next time. It makes it so much  easier to roller paint the roof without vents in the way. I made a rubber gasket to seal the gap. But cleaned of all rust and treated the bare metal first.

[roland elsdon]

Yep done the tapping thing myself. However that's easy if you plan for it. Got horribly caught out doing a favour ( replacing porthole glass) once, when on removing screws realised they were heads only, cut short and ponged in. The porthole being sikaflex on. All of a sudden a quick job  turned into one involving drills taps and threading. Then we realised there was no clearance for the tap  and they were going to cut into the lining, requiring woodwork repairs. A farce and you wondered what else the professional boatbuilder had done. 

[bigcol]

On 30/03/2017 at 17:50, WotEver said:

Alternatively, leave the mushrooms in place, treat the de-rusted area with Vactan, followed by primer, followed by top coat. Ensure that all 3 get into the gap between the vent and the roof where you've damaged the sealant  

Do not use Silicone sealant. 

Hi there

could you let us know,what you meant re not using silicon please why?

[matty40s]

29 minutes ago, bigcol said:

Hi there

could you let us know,what you meant re not using silicon please why?

It makes your hair fall out.☺

[WotEver]

33 minutes ago, bigcol said:

Hi there

could you let us know,what you meant re not using silicon please why?

It doesn't last, not even the supposedly UV stabilised stuff. The acid in it 'may' damage paintwork (lots of anecdotal observations), it creates problems with future paint jobs, it doesn't stay as flexible as marine sealants (such as Sikaflex). 

Edited April 1, 2017 by WotEver
Tryping

[MtB]

Silicone is awful stuff. As WotEver says, only lasts a few years and comes off where you need it to stick, and is a devil to get off when you need to remove it.