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Think of the vacuum bag as a giant sandwich bag (aka "Zip Lock" bag) in which the air is removed by a vacuum source such as an air-powered venturi or an electric vacuum pump. When set up correctly, this creates approximately 1700 lbs per square foot of evenly distributed pressure inside the vacuum bag which is perfect for vacuum veneering.
A vacuum bag typically includes a bag stem and a bag closure system.
Durability and Flexibility
The most durable vacuum bags are made from polyurethane which typically comes in two thicknesses. The 20 mil (.02" thick) is somewhat less durable than the 30 mil simply because of the difference in thickness. Vinyl bags, which are almost always 27 to 30 mil thick, are less durable than polyurethane bags but certainly quite suitable for occasional use by a hobbyist woodworker.
Vinyl is not nearly as flexible as polyurethane but it will conform to large-radius profiles. You'll find that a 30 mil polyurethane bag is somewhat less flexible than a 20 mil polyurethane bag but both sizes are acceptable for tighter-radius profiles.
Selecting a Bag Size
By far, the most common question I receive about vacuum bagging is related to choosing the right size. You have to ask yourself three questions.
What is the largest project size that you can imagine vacuum pressing?
If you can envision your largest veneer project being 3' x 4', you'll probably be very happy with a 4' x 4' vacuum bag.
Does the inconvenience of using a large bag for a small project bother you?
If so, I recommend getting a smaller bag to go along with a larger bag. I use a 4 x 8 vinyl bag for some of my work but I like the convenience and usability of a 2 x 4 polyurethane bag which I use for the largest portion of my furniture projects.
Will your vacuum press system be fast enough for the bag size you need?
Ultimately, the speed of vacuum flow from your vacuum source is what determines the maximum bag size that you should consider. If your system is underpowered, the bag will not pull down fast enough and the glue will set up before full pressure is on the veneer.
Vacuum Flow |
Maximum Bag Size
For Flat Projects |
Maximum Bag Size
for Curved Projects |
1 CFM |
4' x 4' |
2' x 4' |
3 CFM |
4' x 9' |
4' x 4' |
5 CFM |
4' x 15' |
4' x 8' |
An Ounce of Prevention...
A vacuum bag will last much longer if you take good care of it. There are three important and easy to remember aspects of making a vacuum bag last for a very long time.
- The first is to store your vacuum bag out away from UV light sources such as sunlight.
- Make sure there are no sharp edges inside the bag. This includes the platens and the project panels.
- Be careful that you do not stress the seams on your vacuum bag. The seams will surely break if they are being pulled apart by vacuum. With a bag of the correct size, the seams will pull together. Imagine a very small project inside a large bag. Under pressure, you would notice that the seams actually pull together and are under virtually no stress. In the image below, the "Incorrect Bag Use" shows a bag that is too small for the project.
Vacuum Bagging Tips
- Other than inconvenience, there is nothing wrong with using a large bag for a small project.
 If you have a large project going into a large vacuum press bag, consider cutting off the seal on the far end of the bag and using a second bag closure. It is much easier for two people to load the project when one person can help pull it in from the other side.- The VeneerSupplies.com web site has a full selection of vacuum bags and bagging materials.
- Check out the FAQ page for more information about vacuum bags.
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