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Discussion Starter #1 (Edited)
HI friends: I've found myself in a bit of a fan install conundrum. I've got the MaxxFan ready to go, but I am stuck on whether to use the Dicor Lap Sealant or silicone with the butyl tape. After investigating, it looks like the Dicor 501LSW-1 EPDM Self-leveling lap sealant is not rated for use on painted surfaces. So, I guess I would use it only the top screw heads, and silicone the lap.

I thought I was overthinking until I read the MaxxFan instructions which say:

CAUTION: When installing your Roof Flange, only use a caulking/sealant compatible with the flange (plastic product) and the roof surface. DO NOT use materials containing solvents such as or similar to Xylene, Toluene, Methyl Ethyl Ketone, Acetate or Acetone as they can damage the flange. Also, Polysulfide (Thiokol) type sealants must be avoided. Silicone Sealants such as G.E. Silicone II (nonpaintable) used for outdoor applications have proven to be suitable for this installation.

The Dicor website says this:

Dicor’s HAPS Free Self-Leveling Lap Sealant creates a secure, secondary seal along the roof’s edges, air vents, vent pipes and screw heads. Compatible with EPDM, TPO and PVC membranes, it adheres firmly to aluminum, mortar, wood, vinyl, galvanized metal, fiberglass and concrete. Ideally suited for aftermarket use, the lap sealant is one of the highest volume products sold in RV stores today.

I also read some RV forums. My summary is that lap sealant is for fiberglass or aluminum roofed RVs. The MSDS says the main nasty in Dicor is naptha. So I guess the question is whether naptha is in the same league as MEK, toluene and acetone.

Do folks think Dicor is safe to use. I know some RVrs hate silicone, but SFW if that's what MaxxFan specs? Help?

I'll confess a bias. I am not a fan of spritzing Dicor ooze all over my roof. We did it to my son's E250, but that was ugly to begin with. If more is needed to make it work better then you likely have the wrong product. Dicor is one of those things that falls into the "apply liberally" category, and that made me wonder why. I mean, we're not caulking a log cabin or Old Ironsides. The roof fan is a relatively precise install when done well, in all three dimensions.

So please talk me out of MaxxFan's recommended silicone!

How about this stuff: Dynaflex 230 10.1 oz. White Premium Indoor/Outdoor Sealant? I saw it used here.
 

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I've used every sealant known to man on house windows, doors, trim, etc. That experience may or may not be useful for vans. Over the years, I've become less enamored with silicone except maybe for glass, like
showers. I've spent too much time replacing silicone that, in the end, just failed to adhere. I've gravitated toward sealants that are urethane-based because I think they hold up better. That's what I used on my 6" round fan on my Promaster City roof. I sold it after 2 years to buy the Metris,
so, I'm not sure that's a great data point, but it didn't leak. Urethane is the most expensive and messiest sealant, so, if people are putting up with those drawbacks, there must be something good about it, right?
I couldn't find a tube to check for your bad chemicals, but I didn't see any paint issues on mine. I don't know the brand, either. Just urethane.
The other sealant I like is Loctite's Polyseamseal, which is used a great deal for sealing sinks, an application very similar to a roof fan. I have never seen a failure with PSS, but, I've never used it outdoors in sun. It's extremely easy to clean up with water, so, how can that be any good? 😉
That's not a very scientific assessment, but sometimes experience supplements the science. I'm jealous you have the fan. I have to wait til the end of the month for them to be re-stocked. I should be asking these questions in preparation for the same job. I'll be watching to see other comments.
 

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Dynaflex 230 is a good caulk that lasts up to 10 years on houses for window trim. I would try to use something a little better just because you aren't using it in volume on this project, but I have 30-40 tubes or more on my house and I'm not complaining after 8-9 years. Maybe it will last longer than 10, come to think of it. We have a pretty nasty environment given the altitude and resultant UV along with horrendous temperature swings. Out temp today was 92 and we're forecast for 26 Tuesday after 85 Monday. That is hard on sealants.
 

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Discussion Starter #4
Dynaflex 230 is a good caulk that lasts up to 10 years on houses for window trim. I would try to use something a little better just because you aren't using it in volume on this project, but I have 30-40 tubes or more on my house and I'm not complaining after 8-9 years. Maybe it will last longer than 10, come to think of it. We have a pretty nasty environment given the altitude and resultant UV along with horrendous temperature swings. Out temp today was 92 and we're forecast for 26 Tuesday after 85 Monday. That is hard on sealants.
I just found a YouTube with a seemingly experienced installer using silicone?
 

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My impression is that if silicone doesn't stick, it's on a surface that doesn't remain solid (e.g., wood that gets wet and disintegrates or metal that creeps rusts) or the surface hasn't been cleaned thoroughly (thin layer of wax that pulls away). If Maxxfan recommends silicone, why not. Just clean the surfaces well. I usually use a bit of non-environmentally friendly volatile non-polar solvent (like gasoline) which removes contaminants and evaporates to leave a clean surface. Non volatile solvents which remain in the caulking will eat through wax and stick, but may also remove plasticizers in the plastic, causing them to fail eventually.
Just my opinion...
 

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One of the drawbacks of silicone is silicone does not stick to cured silicone, so you can’t use it to reseal the fan once it breaks down. I know very little about sealants on RVs and vans, but I do know that how materials interact is important, and unpainted aluminum or fiberglass are entirely different materially from painted steel. How popular dicor is on fiberglass and unpainted aluminum RV roofs, and how good it is in that application, is irrelevant to how it will work on the painted steel roof of the Metris.

Edit; btw, when you seal a fan or whatever on a hole in an RV roof made of aluminum, fiberglass, or some kind of plastic membrane, you are only concerned with stopping the water from getting into RV through the gaps between the installed device and the roof. On a painted galvanized steel roof, you are also concerned with avoiding electrolyte infused water (ex. Rain) from becoming trapped against the exposed portion of the steel (the part you sawzalled, however thin, is exposed), which will cause the beginning of rust, which will be hidden by the seal until it has taken such hold that you’ll have a very big and expensive problem on your hands.
 

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Discussion Starter #8
One of the drawbacks of silicone is silicone does not stick to cured silicone, so you can’t use it to reseal the fan once it breaks down. I know very little about sealants on RVs and vans, but I do know that how materials interact is important, and unpainted aluminum or fiberglass are entirely different materially from painted steel. How popular dicor is on fiberglass and unpainted aluminum RV roofs, and how good it is in that application, is irrelevant to how it will work on the painted steel roof of the Metris.

Edit; btw, when you seal a fan or whatever on a hole in an RV roof made of aluminum, fiberglass, or some kind of plastic membrane, you are only concerned with stopping the water from getting into RV through the gaps between the installed device and the roof. On a painted galvanized steel roof, you are also concerned with avoiding electrolyte infused water (ex. Rain) from becoming trapped against the exposed portion of the steel (the part you sawzalled, however thin, is exposed), which will cause the beginning of rust, which will be hidden by the seal until it has taken such hold that you’ll have a very big and expensive problem on your hands.
Yes, silicone on silicone is an issue, but I'm talking about the bottom of the mounting plate and not much surface area to strip if ever needed.

I agree edge treatment is important. I don't own a Sawzall, just a drill, center cut shears and a jig saw for this application. On the CRL windows I used etching Rustoleum primer, followed by paint. My ceiling panels are easily removable for inspection in the future.
 
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Discussion Starter #9
My impression is that if silicone doesn't stick, it's on a surface that doesn't remain solid (e.g., wood that gets wet and disintegrates or metal that creeps rusts) or the surface hasn't been cleaned thoroughly (thin layer of wax that pulls away). If Maxxfan recommends silicone, why not. Just clean the surfaces well. I usually use a bit of non-environmentally friendly volatile non-polar solvent (like gasoline) which removes contaminants and evaporates to leave a clean surface. Non volatile solvents which remain in the caulking will eat through wax and stick, but may also remove plasticizers in the plastic, causing them to fail eventually.
Just my opinion...
Yes, I also intend to rough up the painted surface (likely Scotchbrite) before cleaning with VOCs. My base paint has no clear coat, so it doesn't take much prep.
 

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I agree edge treatment is important. I don't own a Sawzall, just a drill, center cut shears and a jig saw for this application. On the CRL windows I used etching Rustoleum primer, followed by paint. My ceiling panels are easily removable for inspection in the future.
I have a sawzall (or rather a large battery powered reciprocating saw Made by Craftsman) and if you want to demolish something quickly but roughly it is an amazing product. Obviously for doing what you are doing a jigsaw is the better tool for the job. I figured you’d know you need to seal the edges, but some people skip that so I thought I’d mention it. The main point about this type of rust is when it occurs it’s extremely insidious.
 

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I'm concerned that Focus805 doesn't own a sawzall. Pictured is the real deal and it sits on the shelf right next to my trim chainsaw. How could one get along without both tools?😀. Notice the fine tooth blades. I'm sure one of those is the culprit that cut through that bundle of pex pipe we had just installed on my son's house remodel project, at 10 P.M on a Sunday night. "Don't worry, son, I can finish up by just cutting this last new heat register hole." 😕
But, forgetting that, which he won't let me, how does one reenact Saturday Night Sawzall Massacre???!!!
IMG_20200905_125040.jpg
 

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I'm concerned that Focus805 doesn't own a sawzall. Pictured is the real deal and it sits on the shelf right next to my trim chainsaw. How could one get along without both tools?😀. Notice the fine tooth blades. I'm sure one of those is the culprit that cut through that bundle of pex pipe we had just installed on my son's house remodel project, at 10 P.M on a Sunday night. "Don't worry, son, I can finish up by just cutting this last new heat register hole." 😕
But, forgetting that, which he won't let me, how does one reenact Saturday Night Sawzall Massacre???!!! View attachment 18082
My first experience with a Sawzall was traumatic -- watching the termite contractor effect repairs on my home.
 

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I've done a fair amount of metal work and air shears work the best. I've a snap on one and cut a access door on my partition and it worked great. You could probably get a electric one from harbor freight for little money. Also, you can buy some metal cutting blades for a jig saw....they work surprisingly well. I have both electric and battery saszall...I wouldn't use it on what your are doing. I've had good luck with clear silicone for keeping out water.
 

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Discussion Starter #14
Yes, thanks. I used HF center cut shears on my window install for the long straight sections. I'm not totally confident with it near corners so I'll do a jig saw, center cut shear hybrid on the roof too, although the horizontal roof cut will be much easier than the sidewall window openings.

I've decided to use marine urethane adhesive to secure the reinforcing frame to the interior ceiling (MaxxFan requires 1 1/8 min. roof depth). My frame will sit 2.75" aft of the second to last horizontal roof brace -- close enough for rigidity and far enough to still let the 8" rear overlap of the MaxxFan sit well fore of the High Rear Brake Light (barn doors). On top, I'll use 3/4" butyl tape on the inside edge of the 1 3/8"-ish MaxxFan plate and apply a siicone bead to the outer edge of the plate, adjacent to but not under the butyl tape. Finally, I'll dutifully coat the top screws and lap the edge with Dicor self-leveling goo, even though it will be ugly and dirty soon.

And seriously, thanks to all for participating in this forum. I have found it extremely useful in my three years of being serious about Metris and nearly two years of ownership.
 
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