No, a paver sealer cannot be used inside of a Chapin Pro Series 4 Gallon Backpack. You may want to contact Chapin directly for advice on which sprayer would be suitable for applying the specific product you have. You can reach them at 800-950-4458.
The Retaining Nut for the Chapin 4 Gallon Backpack Sprayer (Model 61800) is not sold separately. The Chapin Replacement Sprayer Tube (6-8219) includes the wand and the retaining nut.
The Chapin Wand (#6-8141) fits the 61700 SPR 4.0G/15.1L Chapin poly backpack sprayers and the Chapin 4 Gallon Tree/Turf Pro Commercial Backpack Sprayer (61900). We would recommend contacting Chapin directly to troubleshoot and identify specifically what parts are needed for the Chapin Professional 4 Gallon backpack sprayer. Please click the link below to view the replacement parts diagram listed on page 11 of the product manual below.
Chapin can be reached at 1-800-444-3140 for troubleshooting.