Blender object import into Unity messed up

Okay, so when I import my blender object into Unity , a few things messed up

1. some stuff not stick to the bone, so when the character moving, seem like part of the finger is still flying, to fix this i do as below
- a. In blender, top right, click your amature, bottom, click "pose mode" , press "A" on your keyboard to select all bone, bottom, click POSE -> CLEAR TRANSFORM -> ALL
(this will move all the bone back to original place)
- b. Say some of the finger mesh is not attached to the bone, when you still on "POSE MODE" , right click to select your finger bone, then on TOP RIGHT, click your CHARACTER, on BOTTOM, change to "WEIGHT PAINT MODE", you can paint your finger here, but normally you cannot, so you go to "EDIT MODE", click the "LIMIT SELECTION TO VISIBLE" so that you can see through all the face, press "B" then select your entire finger,

- c. go back to "WEIGHT PAINT MODE",  click "FACE SELECTION MASKING FOR PAINTING" then click WEIGHT-> SET WEIGHT

(this will 100% select all the faces for the finger and then set the weight to 100% to attach to the finger bone)

2. The character head is upside down

R T S, rotation, translation, scale

1. create a backup of your blender file
2. open blender and open your file
3. in object mode, select all the meshes (press A), press CONTROL-A (apply menu), then select APPLY ROTATION, then again APPLY TRANSLATION, then APPLY SCALE
4. you'll notice that the objects' pivots will be re-set to the center, and their scale and rotation will be fixed

3. The color is in the wrong direction, say example you build a house, supposingly the color is on outside, but somehow the color is now inside, and outside become transparent

- to fix, this, press 'N' , then under display -> tick the BACKSPACE CURLING, select the face that is wrong direction in "EDIT mode" and hit CTRL-F to Flip them