I have used and been with those that have used PEG-350 and PEG-100. I have never seen them use it on a 7" piece. Burls are often not easy to treat either. What species? That makes a big difference too.
Rockler Woodworking sells it. It is indeed expensive.
Here is an quote from an article I wrote for Sawmill & Woodlot.
Treating Techniques
The concentrated PEG is added to water to get the correct strength. Ten pounds of PEG added to 4.8 quarts of water will result in 7.4 quarts of a 50% solution. The specific gravity (SG) will be 1.093. Measuring the specific gravity during treatment will indicate when more water or more PEG needs to be added to maintain this SG.
It is critical to use water and containers that are iron free, as iron plus tannic acid (common in most woods) will develop iron tannate, a blue-black coloring in or on the wood. If this staining does happen, oxalic acid (wood bleach) can be used to clean the surface.
Because PEG is expensive, it is best to submerge the wood in a tough plastic bag or pail so that there is little excess solution. You do need plenty of solution to keep the piece submerged and in contact with solution on all sides and faces at all times, however. Also, because of the expense, if you are going to work the wood after treating, such as planing or turning or carving, try to do most of this machining before treating, so that you do not treat wood that you will not be using; that is, do not treat the waste wood--treat only what you will be using.
It is also important to understand that some woods do not diffuse moisture or PEG very well. Certainly, white oak, osage orange and locust would not work well. Sometimes the wood in and near the center or core of the tree (the pith) will not treat well; therefore, avoid wood with bad, abnormal-looking cores. In general, lighter weight woods will treat much more thoroughly and much faster. Also, the PEG will enter the end grain faster than the side grain so short pieces will treat much faster than long one.
When treating 1-1/2 thick walnut disks, one of the easier treating species, treating time was 15 days at 70 degrees F and 3 days at 140 degrees; 3 thick, 45 days and 15 days. Due to slow diffusion in many species and the risk of honeycomb when using temperatures over 110 degrees F, it is often impractical to treat wood pieces of the denser woods thicker than 1-1/4. For example, a 1 thick piece of cherry with several holes drilled in it to allow for good penetration took 45 days at 110 degrees.
The U.S. Forest Products Lab states: For cross sections [meaning disks) of lower density woods such as white pine, spruce, redwood, and the soft hardwoods (soft maple, cottonwood, willow, and others), soak for one-half to two-thirds the time suggested for walnut. For the higher density woods (hard maple, yellow birch, beech, the oaks, and apple), double or triple the treating period. Elevated temperatures are normally required for fully effective treatment of very dense woods (such as manzanita, mesquite, desert ironwood, and burls of most species).
In summary, it may take a bit of experimentation to find the best time and temperature treatment for the pieces you are treating.