Everything Bill said plus; Use the tiniest bit of hardener in your mix. You just want it to turn very slightly pink - or you will have no working time. It will kick too fast. If it does, mix another batch; don't try to use it.
Harry, this reminded me of your 1/16 wooden bipe; the Sopwith Camel(?) Shame it's not 1/8 scale to go with this but you could shoot it next to your 'test' 'Brake (1/16); I think both wood models would look smashing together.
Cement will only texture the surface which you don't want. When you sand that, you're making your piece even thinner. Just shoot Future on the bare plastic. Only one step and you have nothing to lose. Then prime.
Important but not exciting... I have been working every day since the last update so here is the important result. The body and floor mate as a unit and the whole assembly mates to the chassis - in a solid and repeatable way. You've seen many prior pictures of the car mocked-up this way but there is a difference now. Many hours of measuring and test fitting improved it all. I went back and made stronger, more accurate locations for all mounting points. I added two new ones (total of 6), and a pair of chassis brackets which now secure the lower front of the body. I made improved 'clamps' to securely hold the floor unit to the body lower flanges. The trunk aligns with the body perfectly. No Pocher screws anywhere; all fasteners are by 6, 2mm studs, tapped and epoxied in place with nuts. The lower front of the body uses 0-80 bolts with nuts through a brass bracket. I will post all the 'fill-in' shots of these parts and assemblies soon. But here is the result. For the first time, the unit sits flush on the chassis and firewall with no bind, twist or warp; it just glides right onto the locating studs and the bolt holes align. A 'tee' pin is seen in the lower front where the bolt goes through the bracket. They fit perfectly with all the other locators in place, a very rewarding feeling. This is what lets you install and remove body work for finish and interior and have it all go back precisely and unseen when carpet and seats are installed.
Another tip... This is primarily for Rolls builders but may work on other hard-roof cars like Alfa Coupe and Bugatti. I needed to work the bottom of the chassis but with the body attached. I've made body mount brackets (will show soon) which bolt to the bottom rails and connect to the new, lower body edge. First attach the body / floor unit to the chassis. I have 2mm studs through the rear floor and 0-80 bolts through the front floor. If you have workstands, remove the rear one and carefully invert the whole car. Rest the roof on a pad and you're done. Solid and safe to measure and attach parts. No front fender though because of the front stand. I knew there was a good reason I whacked the windshield down. Almost as good as a rotisserie...