seadoo speedster reupholstery help

[smether1]

Hi, I am new to this forum and I am reupholstering our boat seats and had some questions that maybe someone could help us with. This is the first time I have sewn with vinyl. I did a lot of research before I started sewing and it has gone ok but I have had a few bumps in the road. A lot of my problems have been with patterning and know how much vinyl to put between the 2 humps on the bench seat.  We refinished the middle part of the seats and it turned out great. we ended up having to put new foam on all three.
http://s725.photobucket.com/user/smether1/media/IMG_0436_zpscbjfxm3o.jpg.html?sort=3&o=6

But now Im ready to do the bench seat and I have to sew 2 seams over the humps and my concern is that I wont put enough vinyl between them and when I go to staple the vinyl that the 2 seams will not end up in the middle of the humps but closer to the  middle seat. I am using joanns marine vinyl. Looking back I should have used the 4-way allsport stretch vinyl it would have been worth the extra money because it would have been easier to work with.

http://s725.photobucket.com/user/smether1/media/IMG_0780_zpsnjdu3p9a.jpg.html?sort=3&o=5

I dont know if I should lay it straight across and mark it for seams or if i should push down a little into the middle seat so it gives me some extra fabric. I watched a video of a guy who had it straight across and used the heat gun a LOT to get the seats to stretch down so he could staple it. My concern with that is I don't want to hurt the integrity of the vinyl stretching it that much. Does anyone have any thoughts on that?

question number 2. I took off the old covers from the back of the seats and cut them up for patterning and thought that I should do a straight piece of fabric for the boxing but the pieces I cut off looked rounded so I thought I would pattern it after that and see how it went. Well it did not fit well. So I added and 1 1\2 inches to the bottom, did the straight boxing and added a little more for seam allowance and it came out way to big. It was also hard to know where to cut the fabric to pull it through to the other side to staple it down. Also the old seats had some kind of interfacing in them that was sewn into it around the edges and melted to the vinyl up top.
http://s725.photobucket.com/user/smether1/media/IMG_0782_zpsofc3bgsp.jpg.html?sort=3&o=3
http://s725.photobucket.com/user/smether1/media/IMG_0783_zpsueykcepg.jpg.html?sort=3&o=2
http://s725.photobucket.com/user/smether1/media/IMG_0785_zpsjzer6njy.jpg.html?sort=3&o=0

I purchased and interfacing that was mold resistant and breathed well but seems to be a lot thicker than the original. The interfacing made it more difficult to sew the vinyl. Do you think I can leave it out or glue it in with upholstery glue?

I will go track down some pictures of my finished seat cover that didnt fit.

[MinUph]

I did one of those a couple of years ago. Came out nice. It has to be one of the hardest things to pattern. First of all Joanns marine vinyl is about the worst I've ever seen. Of course its cheap but that is what it is "cheap" Why take all the time to do a seat for a nice boat and try and save a few bucks in the process?
  Anyway, Put some guide lines on the foam with a sharpie. Cut the vinyl a little over sized and lay it on the seat to make the patterns.  You can place a sand bag on it so it will stay in place. Mark it carefully and transfer the guild lines to the vinyl then do the same for the next piece and transfer the line to these also these lines will be your guide when sewing notch them and alight while sewing. Make the vinyl fit tight. And put the pulls (listings) on at the same time you sew the last piece on so it is in the finish seam.
  If you have to use a heat gun be very careful with that vinyl it will deform easily. But a heat gun might be needed. It will allow the vinyl to stretch a lot. Unless you go past the point of no return with the heat the vinyl should be fine. It isn't going to last but a year or two in exposure anyway.
  My best recommendation is to use a high quality vinyl to start with. A job like this only needs to be once. Good luck.

[Mike]

don't mark the foam with a sharpie it will burn though the vinyl after a short time ask me how I know

I learned this along time ago when I had to redo this bridge twice


all my hash marks bleed through

[chrisberry12]

Use a steamer to soften and stretch the vinyl.