Fail to Prepare and Prepare to Fail (or F2P=P2F)

Vacuum bagging is not a black art, it is the application of physics to achieve extraordinary results which appear completely impossible to the layman - or a black art for short! (just kidding!) You can do some fabulous ambient cure stuff - where the weight of the fabric and resin holds itself down onto the work, but you would struggle to do some of the higher stressed work on a project like this without a vacuum pump and the right sort of consumables - so here's how to give you the idea.

We are going to lay up a couple of layers of reinforcement onto a foam core which will eventually be the replacement piece of deck to make the boat whole again. As the deck was in no fit state to use as a mould, we're using a piece of regular hardboard to cover it and effectively provide the shape. Most importantly to get a good result, do all the leg work first - prepare properly before you mix the resin and you are half way there... remember! F2P = P2F!

1. The hardboard surface is effectively our mould surface, so before applying resin, fabric or anything, ensure the parimeter is clean and airtight (ie is not onto a porous surface) then apply the vacuum tape to make the outer edge of the vacuum bag - so think carefully -it needs to be a good few inches outside of where the repair patch will end - and indeed you need an area within the vac tape to attach the vac pipe - so give some space for that (about a 2" x 2" space clear of the repair and on relatively flat surface). DO NOT REMOVE THE BACKING TAPE OFF OF THE VAC TAPE!!

2. First, cut the reinforcement materials DRY - in this case the top layers of the foam sandwich (we've already built the lower surface and stuck the foam core down) a layer of 86g glass (for toughness) and a layer of 200g carbon for stiffness.

3. Still not ready for that resin - now we cut the consumables : Peel Ply to cover the whole area, then porous (perforated) release film - often known as 'bread-wrap' - this will protect the next layer (the breather fabric) from the excess resin drained off of the laminate. Next comes the breather fabric, then finally the tough, vacuum bag film which will go over the whole job by about 6 inches in every direction (for a virtually flat panel that is - if the repair is 3D, then a much larger vac bag is needed).

Here's the whole stack. Working from the foam upwards - Reinforcements: 86g E-glass Woven, 200g Carbon Woven. Consumables: Peel Ply, Porous Release Film, Breather Fabric, Vacuum Bag Film. If you want to get hold of handy amounts of vac bagging materials have a look here!

4. To start the lay-up, remove the consumables and the reinforcement materials so you can apply resin to one side of the top of the foam, roll out the reinforcements onto the wet resin to stop them distorting. Wet them out with more resin and a brush or roller.

5. With the reinforcements all down and wetted through, lay on the consumables over the wetted out surface in sequence. The peel ply goes first - which will need some straightening and will start to soak up resin almost straight away - best to pull it around with a flat gloved hand on the surface instead of lifting it and reapplying (which can be a nightmare!)

6. This is followed by the porous release, the breather fabric - the breach unit if you are using one, and finally the vacuum bag - which is best initially at least just left to drape over the whole lot.

7. Closing the bag - Working along the edges , remove the tape protector from the vacuum tape and allow the bag film to fall naturally onto it. Then work your way around sealing the bag down with the tape. Pay particular attention to corners, pleats and 'tubes' caused by the bag film overlapping itself. It must be 100% airtight - not just look like it might be and the pump is a very big test.

8. Flash up your pump and connect. It should take just a few seconds to pull the bag down - more than 10 or 15 and you have a BIG leak. With the bag down, look around the edge for smaller leaks - usually audible as a hissing sound. Old wives tales talk of allowing a small leak to prevent all the resin being sucked out of the repair - but then old wives aren't necessarily the best composite engineers! A 100% leak free bag is what you really want and we spend as long as is needed to get one. Trimming the bag tight to the vac tape is good practise to help find leaks...

And voila! A good solid vacuum bag holding our composites down and squishing the resin throughout the job, and blotting out the excess via the peel ply and porous release (look carefully in the pics taken 5 minutes after the pump was started and you can see resin dots on the breather - a very good sign!) You could get a result by loading up the surface with weights - sand bags or similar - but 1 bar of net pressure due to a vacuum bag, even with a few % loss due to small leaks or pump inefficiency is going to produce around 13.5psi - which is a steady 840Kg per Square Foot! - Which would mean an area of deck like this, which was 5ft x 3ft (1500 x 1000) would need 12.6 metric tonnes on it to generate the same pressure! QED.

Take me back to carbonology!